Kinematics
Overview
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...
that describes the motion
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...
of bodies (objects) and systems (groups of objects) without consideration of the forces that cause the motion.
Kinematics is not to be confused with another branch of classical mechanics: analytical dynamics
Analytical dynamics
In classical mechanics, analytical dynamics, or more briefly dynamics, is concerned about the relationship between motion of bodies and its causes, namely the forces acting on the bodies and the properties of the bodies...
(the study of the relationship between the motion of objects and its causes), sometimes subdivided into kinetics
Kinetics (physics)
In physics and engineering, kinetics is a term for the branch of classical mechanics that is concerned with the relationship between the motion of bodies and its causes, namely forces and torques...
(the study of the relation between external forces and motion) and statics
Statics
Statics is the branch of mechanics concerned with the analysis of loads on physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at a constant velocity...
(the study of the relations in a system at equilibrium).
Unanswered Questions
Encyclopedia
Kinematics is the branch of classical mechanics
that describes the motion
of bodies (objects) and systems (groups of objects) without consideration of the forces that cause the motion.
Kinematics is not to be confused with another branch of classical mechanics: analytical dynamics
(the study of the relationship between the motion of objects and its causes), sometimes subdivided into kinetics
(the study of the relation between external forces and motion) and statics
(the study of the relations in a system at equilibrium). Kinematics also differs from dynamics as used in modern-day physics to describe time-evolution of a system.
The term kinematics is less common today than in the past, but still has a role in physics. (See analytical dynamics
for more detail on usage). The term kinematics also finds use in biomechanics
and animal locomotion
. Further, mathematicians that include time
as a parameter in geometry have developed the subject of kinematic geometry.
The simplest application of kinematics is for particle motion, translational or rotational. The next level of complexity comes from the introduction of rigid bodies, which are collections of particles having time invariant distances between themselves. Rigid bodies might undergo translation and rotation or a combination of both. A more complicated case is the kinematics of a system of rigid bodies, which may be linked together by mechanical joints
. Kinematics can be used to find the possible range of motion for a given mechanism
, or, working in reverse, can be used to design a mechanism that has a desired range of motion. The movement of a crane and the oscillations of a piston in an engine are both simple kinematic systems. The crane is a type of open kinematic chain, while the piston is part of a closed four-bar linkage.
refers to a straight trajectory, and a curve
to a trajectory which may have curvature
. In mechanics and kinematics, "line' and "curve" both refer to any trajectory, in particular a line may be a complex curve in space. Any position along a specified trajectory can be described by a single coordinate, the distance traversed along the path, or arc length. The motion of a particle along a trajectory can be described by specifying the time dependence of its position, for example by specification of the arc length locating the particle at each time t. The following words refer to curves and lines:
Consider for example a tower 50 m south from your home. The reference point is home, the distance 50 m and the direction south. If one only says that the tower is 50 m south, the natural question that arises is "from where?" If one says that the tower is southward from your home, the question that arises is "how far?" If one says the tower is 50 m from your home, the question that arises is "in which direction?" Hence, all these three parameters are crucial to defining uniquely the position of a point in space.
Position is usually described by mathematical quantities that have all these three attributes: the most common are vectors and complex numbers. Usually, only vectors are used. For measurement of distances and directions, usually three dimensional coordinate systems are used with the origin coinciding with the reference point. A three-dimensional coordinate system (whose origin coincides with the reference point) with some provision for time measurement is called a reference frame or frame of reference or simply frame. All observations in physics are incomplete without the reference frame being specified.
where x_{A}, y_{A}, and z_{A} are the Cartesian coordinates of the point. The magnitude of the position vector |r| gives the distance between the point A and the origin.
The direction cosines of the position vector provide a quantitative measure of direction.
It is important to note that the position vector of a particle isn't unique. The position vector of a given particle is different relative to different frames of reference.
is a vector describing the difference in position between two points, i.e. it is the change in position the particle undergoes during the time interval. If point A has position r_{A} = (x_{A},y_{A},z_{A}) and point B has position r_{B} = (x_{B},y_{B},z_{B}), the displacement r_{AB} of B from A is given by
Geometrically, displacement is the shortest distance between the points A and B. Displacement, distinct from position vector, is independent of the reference frame. This can be understood as follows: the positions of points is frame dependent, however, the shortest distance between any pair of points is invariant on translation from one frame to another (barring relativistic cases).
quantity, describing the length of the path between two points along which a particle has travelled.
When considering the motion of a particle over time, distance is the length of the particle's path and may be different from displacement, which is the change from its initial position to its final position. For example, a race car
traversing a 10 km closed loop from start to finish travels a distance of 10 km; its displacement, however, is zero because it arrives back at its initial position.
If the position of the particle is known as a function of time , the distance s it travels from time t_{1} to time t_{2} can be found by
The formula utilizes the fact that over an infinitesimal time interval, the magnitude of the displacement equals the distance covered in that interval. This is analogous to the geometric fact that infinitesimal arcs on a curved line coincide with the chord drawn between the ends of the arc itself.
where Δr is the change in position and Δt is the interval of time over which the position changes. The direction of v is same as the direction of the change in position Δr as Δt>0.
Velocity
is the measure of the rate of change in position with respect to time, that is, how the distance of a point changes with each instant of time. Velocity also is a vector. Instantaneous velocity (the velocity at an instant of time) can be defined as the limiting value of average velocity as the time interval Δt becomes smaller and smaller. Both Δr and Δt approach zero but the ratio v approaches a non-zero limit v. That is,
where dr is an infinitesimal
ly small displacement and dt is an infinitesimally small length of time.Because magnitude of dr is necessarily the distance between two infinitesimally spaced points along the trajectory of the point, it is the same as an increment in arc length along the path of the point, customarily denoted ds. As per its definition in the derivative form, velocity can be said to be the time rate of change of position. Further, as dr is tangential to the actual path, so is the velocity.
As a position vector itself is frame dependent, velocity is also dependent on the reference frame.
The speed
of an object is the magnitude |v| of its velocity. It is a scalar quantity:
The distance traveled by a particle over time is a non-decreasing quantity. Hence, ds/dt is non-negative, which implies that speed is also non-negative.
where Δv is the change in velocity and Δt is the interval of time over which velocity changes.
Acceleration
is the vector quantity describing the rate of change with time of velocity. Instantaneous acceleration (the acceleration at an instant of time) is defined as the limiting value of average acceleration as Δt becomes smaller and smaller. Under such a limit, a → a.
where dv is an infinitesimally small change in velocity and dt is an infinitesimally small length of time.
If the acceleration is non-zero but constant, the motion is said to be motion with constant acceleration. On the other hand, if the acceleration is variable, the motion is called motion with variable acceleration. In motion with variable acceleration, the rate of change of acceleration is called the jerk.
.
Integrating acceleration a with respect to time t gives the change in velocity. When acceleration is constant both in direction and in magnitude, the point is said to be undergoing uniformly accelerated motion. In this case, the integral relations can be simplified:
Additional relations between displacement, velocity, acceleration, and time can be derived. Since ,
By using the definition of an average
, this equation states that when the acceleration is constant average velocity times time equals displacement.
A relationship without explicit time dependence may also be derived for one-dimensional motion. Noting that ,
where · denotes the dot product
. Dividing the t on both sides and carrying out the dot-products:
In the case of straight-line motion, (r - r_{0}) is parallel to a. Then
This relation is useful when time is not known explicitly.
Consequently, the position of A relative to B is
The above relative equation states that the motion of A relative to B is equal to the motion of A relative to O minus the motion of B relative to O. It may be easier to visualize this result if the terms are re-arranged:
or, in words, the motion of A relative to the reference is that of B plus the relative motion of A with respect to B. These relations between displacements become relations between velocities by simple time-differentiation, and a second differentiation makes them apply to accelerations.
For example, let Ann move with velocity relative to the reference (we drop the O subscript for convenience) and let Bob move with velocity , each velocity given with respect to the ground (point O). To find how fast Ann is moving relative to Bob (we call this velocity ), the equation above gives:
To find we simply rearrange this equation to obtain:
At velocities comparable to the speed of light
, these equations are not valid. They are replaced by equations derived from Einstein's theory of special relativity
.
Kinematics is the study of how things move. Here, we are interested in the motion of normal objects in our world. A normal object is visible, has edges, and has a location that can be expressed with (x, y, z) coordinates. We will not be discussing the motion of atomic particles or black holes or light.
We will create a vocabulary and a group of mathematical methods that will describe this ordinary motion. Understand that we will be developing a language for describing motion only. We won't be concerned with what is causing or changing the motion, or more correctly, the momentums of the objects. In other words, we are not concerned with the action of forces within this topic.
and the kinematics of turns induced by algebraic products.
In what follows, attention is restricted to simple rotation about an axis of fixed orientation. The z-axis has been chosen for convenience.
Description of rotation then involves these three quantities:
The angular velocity is represented in Figure 1 by a vector Ω pointing along the axis of rotation with magnitude ω and sense determined by the direction of rotation as given by the right-hand rule
.
The equations of translational kinematics can easily be extended to planar rotational kinematics with simple variable exchanges:
Here θ_{i} and θ_{f} are, respectively, the initial and final angular positions, ω_{i} and ω_{f} are, respectively, the initial and final angular velocities, and α is the constant angular acceleration. Although position in space and velocity in space are both true vectors (in terms of their properties under rotation), as is angular velocity, angle itself is not a true vector.
Displacement. An object in circular motion is located at a position r(t) given by:
where u_{R} is a unit vector pointing outward from the axis of rotation toward the periphery of the circle of motion, located at a radius R from the axis.
Linear velocity. The velocity of the object is then
The magnitude of the unit vector u_{R} (by definition) is fixed, so its time dependence is entirely due to its rotation with the radius to the object, that is,
where u_{θ} is a unit vector perpendicular to u_{R} pointing in the direction of rotation, ω(t) is the (possibly time varying) angular rate of rotation, and the symbol × denotes the vector cross product. The velocity is then:
The velocity therefore is tangential to the circular orbit of the object, pointing in the direction of rotation, and increasing in time if ω increases in time.
Linear acceleration. In the same manner, the acceleration of the object is defined as:
which shows a leading term a_{θ} in the acceleration tangential to the orbit related to the angular acceleration of the object (supposing ω to vary in time) and a second term a_{R} directed inward from the object toward the center of rotation, called the centripetal acceleration
.
s on the motion, or by the geometrical nature of the force causing or affecting the motion. Thus, to describe the motion of a bead constrained to move along a circular hoop, the most useful coordinate may be its angle on the hoop. Similarly, to describe the motion of a particle acted upon by a central force, the most useful coordinates may be polar coordinates. Polar coordinates are extended into three dimensions with either the spherical polar
or cylindrical polar
coordinate systems. These are most useful in systems exhibiting spherical or cylindrical symmetry respectively.
. Sometimes the conventions , , and or , , and are used instead and understood equivalently.
The position
vector, , the velocity
vector, , and the acceleration
vector, are expressed using rectangular coordinates in the following manner. Here we denote first and second derivative with the single dot and double dot above, respectively. In Leibniz's notation, some equivalents would be and for instance.
for the plane in which the objects we are considering reside), and about which we can allow for rotation to occur. For any position vector , we will define to be the unit vector parallel to and to be 90 degrees counterclockwise of , both in the x-y plane. One could also extend this idea to a coordinate system that has a mobile origin that can translate as well as let is unit vectors rotate about. This is particularly useful when we wish to "follow"a particle on a body that is being studied. (However, for the purposes of this article, we will stick with a simple rotation through the x-y plane)
Let us define the orientation-variable basis vectors and in terms of our standard static orthonormal basis vectors and . Let the angle with the x-axis that forms be . Thus, by how we have defined and , we can express the following.
Let us now take the derivative
of each with respect to time (which we will denote with a dot above).
And now we will back-substitute the original definitions of and to eliminate the need to express in terms of and .
Given these identities, we can now figure out how to represent the position, velocity, and acceleration vectors of a particle using this coordinate system
.
It is just the distance from the origin in the direction which we have defined to be always in the direction of (i.e. - adjusts direction as adjusts direction).
since both and could be changing with respect to time.
Substituting in our found value for gives the following.
We can interpret , by definition, as a radial velocity
as well as interpret , by definition, as an angular velocity
. Thus, we can equivalently express the following.
Since the relationship between angular velocity
and tangential velocity is , we can simplify even further into the following trivial form.
multiple times since all the scalar and unit vector quantities could be changing with respect to time.
Substituting our found values for and gives the following.
Collecting unit vector coefficients gives the following expression.
Similarly, we can, by definition, interpret as radial acceleration and where is angular acceleration
as tangential acceleration .
Unlike our expression for velocity, the and components have terms in addition to the trivial notions of radial and tangential acceleration. These are commonly perceived as fictitious forces. Specifically accounts for the centripetal acceleration (equivalent to and ) and accounts for the Coriolis acceleration (equivalent to and ).
without slipping obeys the condition that the velocity
of its center of mass
is equal to the cross product
of its angular velocity
with a vector from the point of contact to the center of mass,.
For the case of an object that does not tip or turn, this reduces to v = R ω.
. Another example is a drum turned by the pull of gravity upon a falling weight attached to the rim by the inextensible cord. An equilibrium problem (not kinematic) of this type is the catenary
.
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...
that describes the motion
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...
of bodies (objects) and systems (groups of objects) without consideration of the forces that cause the motion.
Kinematics is not to be confused with another branch of classical mechanics: analytical dynamics
Analytical dynamics
In classical mechanics, analytical dynamics, or more briefly dynamics, is concerned about the relationship between motion of bodies and its causes, namely the forces acting on the bodies and the properties of the bodies...
(the study of the relationship between the motion of objects and its causes), sometimes subdivided into kinetics
Kinetics (physics)
In physics and engineering, kinetics is a term for the branch of classical mechanics that is concerned with the relationship between the motion of bodies and its causes, namely forces and torques...
(the study of the relation between external forces and motion) and statics
Statics
Statics is the branch of mechanics concerned with the analysis of loads on physical systems in static equilibrium, that is, in a state where the relative positions of subsystems do not vary over time, or where components and structures are at a constant velocity...
(the study of the relations in a system at equilibrium). Kinematics also differs from dynamics as used in modern-day physics to describe time-evolution of a system.
The term kinematics is less common today than in the past, but still has a role in physics. (See analytical dynamics
Analytical dynamics
In classical mechanics, analytical dynamics, or more briefly dynamics, is concerned about the relationship between motion of bodies and its causes, namely the forces acting on the bodies and the properties of the bodies...
for more detail on usage). The term kinematics also finds use in biomechanics
Biomechanics
Biomechanics is the application of mechanical principles to biological systems, such as humans, animals, plants, organs, and cells. Perhaps one of the best definitions was provided by Herbert Hatze in 1974: "Biomechanics is the study of the structure and function of biological systems by means of...
and animal locomotion
Animal locomotion
Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...
. Further, mathematicians that include time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
as a parameter in geometry have developed the subject of kinematic geometry.
The simplest application of kinematics is for particle motion, translational or rotational. The next level of complexity comes from the introduction of rigid bodies, which are collections of particles having time invariant distances between themselves. Rigid bodies might undergo translation and rotation or a combination of both. A more complicated case is the kinematics of a system of rigid bodies, which may be linked together by mechanical joints
Linkage (mechanical)
A mechanical linkage is an assembly of bodies connected together to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid. The connections between links are modeled as providing ideal movement, pure rotation or sliding for...
. Kinematics can be used to find the possible range of motion for a given mechanism
Mechanism (engineering)
A mechanism is a device designed to transform input forces and movement into a desired set of output forces and movement. Mechanisms generally consist of moving components such as gears and gear trains, belt and chain drives, cam and follower mechanisms, and linkages as well as friction devices...
, or, working in reverse, can be used to design a mechanism that has a desired range of motion. The movement of a crane and the oscillations of a piston in an engine are both simple kinematic systems. The crane is a type of open kinematic chain, while the piston is part of a closed four-bar linkage.
Linear motion
Linear or translational kinematics is the description of the motion in space of a point along a line, also known as a trajectory or path.In mathematics, a lineLine (geometry)
The notion of line or straight line was introduced by the ancient mathematicians to represent straight objects with negligible width and depth. Lines are an idealization of such objects...
refers to a straight trajectory, and a curve
Curve
In mathematics, a curve is, generally speaking, an object similar to a line but which is not required to be straight...
to a trajectory which may have curvature
Differential geometry of curves
Differential geometry of curves is the branch of geometry that dealswith smooth curves in the plane and in the Euclidean space by methods of differential and integral calculus....
. In mechanics and kinematics, "line' and "curve" both refer to any trajectory, in particular a line may be a complex curve in space. Any position along a specified trajectory can be described by a single coordinate, the distance traversed along the path, or arc length. The motion of a particle along a trajectory can be described by specifying the time dependence of its position, for example by specification of the arc length locating the particle at each time t. The following words refer to curves and lines:
- "linear" (= along a straight or curved line;
- "rectilinear" (= along a straight line, from Latin rectus = straight, and linere = spread),
- "curvilinear" (=along a curved line, from Latin curvus = curved, and linere = spread). This path can be either straight (rectilinear) or curved (curvilinear).
Particle kinematics
Particle kinematics is the study of the kinematics of a single particle. The results obtained in particle kinematics are used to study the kinematics of collection of particles, dynamics and in many other branches of mechanics.Position and reference frames
The position of a point in space is the most fundamental idea in particle kinematics. To specify the position of a point, one must specify three things: the reference point (often called the origin), distance from the reference point and the direction in space of the straight line from the reference point to the particle. Exclusion of any of these three parameters renders the description of position incomplete.Consider for example a tower 50 m south from your home. The reference point is home, the distance 50 m and the direction south. If one only says that the tower is 50 m south, the natural question that arises is "from where?" If one says that the tower is southward from your home, the question that arises is "how far?" If one says the tower is 50 m from your home, the question that arises is "in which direction?" Hence, all these three parameters are crucial to defining uniquely the position of a point in space.
Position is usually described by mathematical quantities that have all these three attributes: the most common are vectors and complex numbers. Usually, only vectors are used. For measurement of distances and directions, usually three dimensional coordinate systems are used with the origin coinciding with the reference point. A three-dimensional coordinate system (whose origin coincides with the reference point) with some provision for time measurement is called a reference frame or frame of reference or simply frame. All observations in physics are incomplete without the reference frame being specified.
Position vector
The position vector of a particle is a vector drawn from the origin of the reference frame to the particle. It expresses both the distance of the point from the origin and its sense from the origin. In three dimensions, the position of point A can be expressed aswhere x_{A}, y_{A}, and z_{A} are the Cartesian coordinates of the point. The magnitude of the position vector |r| gives the distance between the point A and the origin.
The direction cosines of the position vector provide a quantitative measure of direction.
It is important to note that the position vector of a particle isn't unique. The position vector of a given particle is different relative to different frames of reference.
Rest and motion
Once the concept of position is firmly established, the ideas of rest and motion naturally follow. If the position vector of the particle (relative to a given reference frame) changes with time, then the particle is said to be in motion with respect to the chosen reference frame. However, if the position vector of the particle (relative to a given reference frame) remains the same with time, then the particle is said to be at rest with respect to the chosen frame. Note that rest and motion are relative to the reference frame chosen. It is quite possible that a particle at rest relative to a particular reference frame is in motion relative to the other. Hence, rest and motion aren't absolute terms, rather they are dependent on reference frame. For example, a passenger in a moving car may be at rest with respect to the car, but in motion with respect to the road.Path
A particle's path is the locus between its beginning and end points which is reference-frame dependent. The path of a particle may be rectilinear (straight line) in one frame, and curved in another.Displacement
DisplacementDisplacement (vector)
A displacement is the shortest distance from the initial to the final position of a point P. Thus, it is the length of an imaginary straight path, typically distinct from the path actually travelled by P...
is a vector describing the difference in position between two points, i.e. it is the change in position the particle undergoes during the time interval. If point A has position r_{A} = (x_{A},y_{A},z_{A}) and point B has position r_{B} = (x_{B},y_{B},z_{B}), the displacement r_{AB} of B from A is given by
Geometrically, displacement is the shortest distance between the points A and B. Displacement, distinct from position vector, is independent of the reference frame. This can be understood as follows: the positions of points is frame dependent, however, the shortest distance between any pair of points is invariant on translation from one frame to another (barring relativistic cases).
Distance
Distance is a scalarScalar (physics)
In physics, a scalar is a simple physical quantity that is not changed by coordinate system rotations or translations , or by Lorentz transformations or space-time translations . This is in contrast to a vector...
quantity, describing the length of the path between two points along which a particle has travelled.
When considering the motion of a particle over time, distance is the length of the particle's path and may be different from displacement, which is the change from its initial position to its final position. For example, a race car
Auto racing
Auto racing is a motorsport involving the racing of cars for competition. It is one of the world's most watched televised sports.-The beginning of racing:...
traversing a 10 km closed loop from start to finish travels a distance of 10 km; its displacement, however, is zero because it arrives back at its initial position.
If the position of the particle is known as a function of time , the distance s it travels from time t_{1} to time t_{2} can be found by
The formula utilizes the fact that over an infinitesimal time interval, the magnitude of the displacement equals the distance covered in that interval. This is analogous to the geometric fact that infinitesimal arcs on a curved line coincide with the chord drawn between the ends of the arc itself.
Velocity and speed
Average velocity is defined aswhere Δr is the change in position and Δt is the interval of time over which the position changes. The direction of v is same as the direction of the change in position Δr as Δt>0.
Velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
is the measure of the rate of change in position with respect to time, that is, how the distance of a point changes with each instant of time. Velocity also is a vector. Instantaneous velocity (the velocity at an instant of time) can be defined as the limiting value of average velocity as the time interval Δt becomes smaller and smaller. Both Δr and Δt approach zero but the ratio v approaches a non-zero limit v. That is,
where dr is an infinitesimal
Infinitesimal
Infinitesimals have been used to express the idea of objects so small that there is no way to see them or to measure them. The word infinitesimal comes from a 17th century Modern Latin coinage infinitesimus, which originally referred to the "infinite-th" item in a series.In common speech, an...
ly small displacement and dt is an infinitesimally small length of time.Because magnitude of dr is necessarily the distance between two infinitesimally spaced points along the trajectory of the point, it is the same as an increment in arc length along the path of the point, customarily denoted ds. As per its definition in the derivative form, velocity can be said to be the time rate of change of position. Further, as dr is tangential to the actual path, so is the velocity.
As a position vector itself is frame dependent, velocity is also dependent on the reference frame.
The speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
of an object is the magnitude |v| of its velocity. It is a scalar quantity:
The distance traveled by a particle over time is a non-decreasing quantity. Hence, ds/dt is non-negative, which implies that speed is also non-negative.
Acceleration
Average acceleration (acceleration over a length of time) is defined as:where Δv is the change in velocity and Δt is the interval of time over which velocity changes.
Acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
is the vector quantity describing the rate of change with time of velocity. Instantaneous acceleration (the acceleration at an instant of time) is defined as the limiting value of average acceleration as Δt becomes smaller and smaller. Under such a limit, a → a.
where dv is an infinitesimally small change in velocity and dt is an infinitesimally small length of time.
Types of motion based on velocity and acceleration
If the acceleration of a particle is zero, then the velocity of the particle is constant over time and the motion is said to be uniform. Otherwise, the motion is non-uniform.If the acceleration is non-zero but constant, the motion is said to be motion with constant acceleration. On the other hand, if the acceleration is variable, the motion is called motion with variable acceleration. In motion with variable acceleration, the rate of change of acceleration is called the jerk.
Integral relations
The above definitions can be inverted by mathematical integration to find:Kinematics of constant acceleration
Many physical situations can be modeled as constant-acceleration processes, such as projectile motionProjectile motion
The motion in which a body is thrown or projected is called Projectile motion.The path followed by a projectile is called its trajectory, which is directly influenced by gravity....
.
Integrating acceleration a with respect to time t gives the change in velocity. When acceleration is constant both in direction and in magnitude, the point is said to be undergoing uniformly accelerated motion. In this case, the integral relations can be simplified:
Additional relations between displacement, velocity, acceleration, and time can be derived. Since ,
By using the definition of an average
Average
In mathematics, an average, or central tendency of a data set is a measure of the "middle" value of the data set. Average is one form of central tendency. Not all central tendencies should be considered definitions of average....
, this equation states that when the acceleration is constant average velocity times time equals displacement.
A relationship without explicit time dependence may also be derived for one-dimensional motion. Noting that ,
where · denotes the dot product
Dot product
In mathematics, the dot product or scalar product is an algebraic operation that takes two equal-length sequences of numbers and returns a single number obtained by multiplying corresponding entries and then summing those products...
. Dividing the t on both sides and carrying out the dot-products:
In the case of straight-line motion, (r - r_{0}) is parallel to a. Then
This relation is useful when time is not known explicitly.
Relative velocity
To describe the motion of object A with respect to object B, when we know how each is moving with respect to a reference object O, we can use vector algebra. Choose an origin for reference, and let the positions of objects A, B, and O be denoted by r_{A}, r_{B}, and r_{O}. Then the position of A relative to the reference object O isConsequently, the position of A relative to B is
The above relative equation states that the motion of A relative to B is equal to the motion of A relative to O minus the motion of B relative to O. It may be easier to visualize this result if the terms are re-arranged:
or, in words, the motion of A relative to the reference is that of B plus the relative motion of A with respect to B. These relations between displacements become relations between velocities by simple time-differentiation, and a second differentiation makes them apply to accelerations.
For example, let Ann move with velocity relative to the reference (we drop the O subscript for convenience) and let Bob move with velocity , each velocity given with respect to the ground (point O). To find how fast Ann is moving relative to Bob (we call this velocity ), the equation above gives:
To find we simply rearrange this equation to obtain:
At velocities comparable to the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
, these equations are not valid. They are replaced by equations derived from Einstein's theory of special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...
.
Example: Rectilinear (1D) motion |
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Consider an object that is fired directly upwards and falls back to the ground so that its trajectory is contained in a straight line. If we adopt the convention that the upward direction is the positive direction, the object experiences a constant acceleration of approximately −9.81 m s^{−2}. Therefore, its motion can be modeled with the equations governing uniformly accelerated motion. For the sake of example, assume the object has an initial velocity of +50 m s^{−1}. There are several interesting kinematic questions we can ask about the particle's motion: How long will it be airborne? To answer this question, we apply the formula Since the question asks for the length of time between the object leaving the ground and hitting the ground on its fall, the displacement is zero. There are two solutions: the first, , is trivial. The solution of interest is What altitude will it reach before it begins to fall? In this case, we use the fact that the object has a velocity of zero at the apex of its trajectory. Therefore, the applicable equation is: If the origin of our coordinate system is at the ground, then x_{i} is zero. Then we solve for x_{f} and substitute known values: What will its final velocity be when it reaches the ground? To answer this question, we use the fact that the object has an initial velocity of zero at the apex before it begins its descent. We can use the same equation we used for the last question, using the value of 127.55 m for x_{i}. Assuming this experiment were performed in a vacuum (negating drag effects), we find that the final and initial speeds are equal, a result which agrees with conservation of energy Conservation of energy The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time... . |
Example: Projectile (2D) motion |
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Suppose that an object is not fired vertically but is fired at an angle θ from the ground. The object will then follow a parabolic trajectory, and its horizontal motion can be modeled independently of its vertical motion. Assume that the object is fired at an initial velocity of 50 m s^{−1} and 30° from the horizontal. How far will it travel before hitting the ground? The object experiences an acceleration of −9.81 m s^{−2} in the vertical direction and no acceleration in the horizontal direction. Therefore, the horizontal displacement is Solving the equation requires finding t. This can be done by analyzing the motion in the vertical direction. If we impose that the vertical displacement is zero, we can use the same procedure we did for rectilinear motion to find t. We now solve for t and substitute this expression into the original expression for horizontal displacement. Note the use of the trigonometric identity . |
Kinematics is the study of how things move. Here, we are interested in the motion of normal objects in our world. A normal object is visible, has edges, and has a location that can be expressed with (x, y, z) coordinates. We will not be discussing the motion of atomic particles or black holes or light.
We will create a vocabulary and a group of mathematical methods that will describe this ordinary motion. Understand that we will be developing a language for describing motion only. We won't be concerned with what is causing or changing the motion, or more correctly, the momentums of the objects. In other words, we are not concerned with the action of forces within this topic.
Rotational motion
Rotational or angular kinematics is the description of the rotation of an object. The description of rotation requires some method for describing orientation. Common descriptions include Euler anglesEuler angles
The Euler angles are three angles introduced by Leonhard Euler to describe the orientation of a rigid body. To describe such an orientation in 3-dimensional Euclidean space three parameters are required...
and the kinematics of turns induced by algebraic products.
In what follows, attention is restricted to simple rotation about an axis of fixed orientation. The z-axis has been chosen for convenience.
Description of rotation then involves these three quantities:
- Angular position: The oriented distance from a selected origin on the rotational axis to a point of an object is a vector r ( t ) locating the point. The vector r(t) has some projection (or, equivalently, some component) r_{⊥}(t) on a plane perpendicular to the axis of rotation. Then the angular position of that point is the angle θ from a reference axis (typically the positive x-axis) to the vector r_{⊥}(t) in a known rotation sense (typically given by the right-hand ruleRight-hand ruleIn mathematics and physics, the right-hand rule is a common mnemonic for understanding notation conventions for vectors in 3 dimensions. It was invented for use in electromagnetism by British physicist John Ambrose Fleming in the late 19th century....
). - Angular velocity: The angular velocity ω is the rate at which the angular position θ changes with respect to time t:
The angular velocity is represented in Figure 1 by a vector Ω pointing along the axis of rotation with magnitude ω and sense determined by the direction of rotation as given by the right-hand rule
Right-hand rule
In mathematics and physics, the right-hand rule is a common mnemonic for understanding notation conventions for vectors in 3 dimensions. It was invented for use in electromagnetism by British physicist John Ambrose Fleming in the late 19th century....
.
- Angular acceleration: The magnitude of the angular acceleration α is the rate at which the angular velocity ω changes with respect to time t:
The equations of translational kinematics can easily be extended to planar rotational kinematics with simple variable exchanges:
Here θ_{i} and θ_{f} are, respectively, the initial and final angular positions, ω_{i} and ω_{f} are, respectively, the initial and final angular velocities, and α is the constant angular acceleration. Although position in space and velocity in space are both true vectors (in terms of their properties under rotation), as is angular velocity, angle itself is not a true vector.
Point object in circular motion
This example deals with a "point" object, by which is meant that complications due to rotation of the body itself about its own center of mass are ignored.Displacement. An object in circular motion is located at a position r(t) given by:
where u_{R} is a unit vector pointing outward from the axis of rotation toward the periphery of the circle of motion, located at a radius R from the axis.
Linear velocity. The velocity of the object is then
The magnitude of the unit vector u_{R} (by definition) is fixed, so its time dependence is entirely due to its rotation with the radius to the object, that is,
where u_{θ} is a unit vector perpendicular to u_{R} pointing in the direction of rotation, ω(t) is the (possibly time varying) angular rate of rotation, and the symbol × denotes the vector cross product. The velocity is then:
The velocity therefore is tangential to the circular orbit of the object, pointing in the direction of rotation, and increasing in time if ω increases in time.
Linear acceleration. In the same manner, the acceleration of the object is defined as:
which shows a leading term a_{θ} in the acceleration tangential to the orbit related to the angular acceleration of the object (supposing ω to vary in time) and a second term a_{R} directed inward from the object toward the center of rotation, called the centripetal acceleration
Centripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...
.
Coordinate systems
In any given situation, the most useful coordinates may be determined by constraintConstraint (mathematics)
In mathematics, a constraint is a condition that a solution to an optimization problem must satisfy. There are two types of constraints: equality constraints and inequality constraints...
s on the motion, or by the geometrical nature of the force causing or affecting the motion. Thus, to describe the motion of a bead constrained to move along a circular hoop, the most useful coordinate may be its angle on the hoop. Similarly, to describe the motion of a particle acted upon by a central force, the most useful coordinates may be polar coordinates. Polar coordinates are extended into three dimensions with either the spherical polar
Spherical coordinate system
In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the radial distance of that point from a fixed origin, its inclination angle measured from a fixed zenith direction, and the azimuth angle of...
or cylindrical polar
Cylindrical coordinate system
A cylindrical coordinate system is a three-dimensional coordinate systemthat specifies point positions by the distance from a chosen reference axis, the direction from the axis relative to a chosen reference direction, and the distance from a chosen reference plane perpendicular to the axis...
coordinate systems. These are most useful in systems exhibiting spherical or cylindrical symmetry respectively.
Fixed rectangular coordinates
In fixed rectangular coordinates, vectors are expressed as an addition of vectors in the x, y, and z direction from a non-rotating origin. Usually , , and are unit vectors in the x-, y-, and z-directions forming an orthonormal basisOrthonormal basis
In mathematics, particularly linear algebra, an orthonormal basis for inner product space V with finite dimension is a basis for V whose vectors are orthonormal. For example, the standard basis for a Euclidean space Rn is an orthonormal basis, where the relevant inner product is the dot product of...
. Sometimes the conventions , , and or , , and are used instead and understood equivalently.
The position
Position
Position may refer to:* Position , a player role within a team* Position , the orientation of a baby prior to birth* Position , a mathematical identification of relative location...
vector, , the velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
vector, , and the acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
vector, are expressed using rectangular coordinates in the following manner. Here we denote first and second derivative with the single dot and double dot above, respectively. In Leibniz's notation, some equivalents would be and for instance.
Two-Dimensional Rotating/Polar Coordinates
The coordinate system we will study here system expresses motion only along the x-y plane. It is based on three orthogonal unit vectors: the vector , and the vector (which form an orthonormal basisOrthonormal basis
In mathematics, particularly linear algebra, an orthonormal basis for inner product space V with finite dimension is a basis for V whose vectors are orthonormal. For example, the standard basis for a Euclidean space Rn is an orthonormal basis, where the relevant inner product is the dot product of...
for the plane in which the objects we are considering reside), and about which we can allow for rotation to occur. For any position vector , we will define to be the unit vector parallel to and to be 90 degrees counterclockwise of , both in the x-y plane. One could also extend this idea to a coordinate system that has a mobile origin that can translate as well as let is unit vectors rotate about. This is particularly useful when we wish to "follow"a particle on a body that is being studied. (However, for the purposes of this article, we will stick with a simple rotation through the x-y plane)
Derivatives of unit vectors
The position , velocity , and acceleration vectors of a given point can be expressed using and , but we have to take care to keep track of how these basis vectors change over time. For instance, when the reference frame rotates, the unit basis vectors also rotate, and this rotation must be taken into account when take time derivatives of to find and .Let us define the orientation-variable basis vectors and in terms of our standard static orthonormal basis vectors and . Let the angle with the x-axis that forms be . Thus, by how we have defined and , we can express the following.
Let us now take the derivative
Derivative
In calculus, a branch of mathematics, the derivative is a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much one quantity is changing in response to changes in some other quantity; for example, the derivative of the position of a...
of each with respect to time (which we will denote with a dot above).
And now we will back-substitute the original definitions of and to eliminate the need to express in terms of and .
Given these identities, we can now figure out how to represent the position, velocity, and acceleration vectors of a particle using this coordinate system
Coordinate system
In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element. The order of the coordinates is significant and they are sometimes identified by their position in an ordered tuple and sometimes by...
.
Position
Position is straightforward:It is just the distance from the origin in the direction which we have defined to be always in the direction of (i.e. - adjusts direction as adjusts direction).
Velocity
Velocity is the time derivative of position, which we will apply via the product ruleProduct rule
In calculus, the product rule is a formula used to find the derivatives of products of two or more functions. It may be stated thus:'=f'\cdot g+f\cdot g' \,\! or in the Leibniz notation thus:...
since both and could be changing with respect to time.
Substituting in our found value for gives the following.
We can interpret , by definition, as a radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
as well as interpret , by definition, as an angular velocity
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...
. Thus, we can equivalently express the following.
Since the relationship between angular velocity
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...
and tangential velocity is , we can simplify even further into the following trivial form.
Acceleration
Acceleration is the time derivative of velocity, which we will apply via the product ruleProduct rule
In calculus, the product rule is a formula used to find the derivatives of products of two or more functions. It may be stated thus:'=f'\cdot g+f\cdot g' \,\! or in the Leibniz notation thus:...
multiple times since all the scalar and unit vector quantities could be changing with respect to time.
Substituting our found values for and gives the following.
Collecting unit vector coefficients gives the following expression.
Similarly, we can, by definition, interpret as radial acceleration and where is angular acceleration
Angular acceleration
Angular acceleration is the rate of change of angular velocity over time. In SI units, it is measured in radians per second squared , and is usually denoted by the Greek letter alpha .- Mathematical definition :...
as tangential acceleration .
Unlike our expression for velocity, the and components have terms in addition to the trivial notions of radial and tangential acceleration. These are commonly perceived as fictitious forces. Specifically accounts for the centripetal acceleration (equivalent to and ) and accounts for the Coriolis acceleration (equivalent to and ).
Kinematic constraints
A kinematic constraint is any condition relating properties of a dynamic system that must hold true at all times. Below are some common examples:Rolling without slipping
An object that rolls against a surfaceSurface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
without slipping obeys the condition that the velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of its center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
is equal to the cross product
Cross product
In mathematics, the cross product, vector product, or Gibbs vector product is a binary operation on two vectors in three-dimensional space. It results in a vector which is perpendicular to both of the vectors being multiplied and normal to the plane containing them...
of its angular velocity
Angular velocity
In physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...
with a vector from the point of contact to the center of mass,.
For the case of an object that does not tip or turn, this reduces to v = R ω.
Inextensible cord
This is the case where bodies are connected by an idealized cord that remains in tension and cannot change length. The constraint is that the sum of lengths of all segments of the cord is the total length, and accordingly the time derivative of this sum is zero. See Kelvin and Tait and Fogiel. A dynamic problem of this type is the pendulumPendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...
. Another example is a drum turned by the pull of gravity upon a falling weight attached to the rim by the inextensible cord. An equilibrium problem (not kinematic) of this type is the catenary
Catenary
In physics and geometry, the catenary is the curve that an idealised hanging chain or cable assumes when supported at its ends and acted on only by its own weight. The curve is the graph of the hyperbolic cosine function, and has a U-like shape, superficially similar in appearance to a parabola...
.
External links
- Java applet of 1D kinematics
- Flash animated tutorial for 1D kinematics
- Physclips: Mechanics with animations and video clips from the University of New South Wales
- physicsfunda.googlepages.com, Kinematics for High School ant IIT JEE level
- Kinematic Models for Design Digital Library (KMODDL)
Movies and photos of hundreds of working mechanical-systems models at Cornell UniversityCornell UniversityCornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...
. Also includes an e-book library of classic texts on mechanical design and engineering.