Ground reaction force
Encyclopedia
In physics
, and in particular in biomechanics
, the ground reaction force (GRF) is the force exerted by the ground on a body in contact with it.
For example, a person standing on the ground exerts a contact force
on it (equal to the person's weight
) and at the same time an equal and opposite ground reaction force is exerted by the ground on the person.
In the mentioned example, the ground reaction force coincides with the notion of normal force
. However, in a more general case, the GRF will also have a component parallel to the ground, for example when the person is walking, which requires exchanging horizontal forces with the ground in order to achieve motion.
The use of the word reaction
derives from Newton's third law, which essentially states that if a force, called action, acts upon a body, then an equal and opposite force, called reaction, must act upon another body. The force exerted by the ground is commonly referred to as the reaction, although, since the distinction between action and reaction is completely arbitrary, the expression ground action would be, in principle, equally acceptable.
Ground Reaction Force is not equivalent to friction, however the normal component of the ground reaction force (GRF) can contribute to friction: the maximum force of friction between two surfaces equals the product of normal force, or the component of force applied perpendicular to the contact surface, and the coefficient of friction.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, and in particular 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...
, the ground reaction force (GRF) is the force exerted by the ground on a body in contact with it.
For example, a person standing on the ground exerts a contact force
Contact force
In physics, a contact force is a force that acts at the point of contact between two objects, in contrast to body forces. Contact forces are described by Newton's laws of motion, as with all other forces in dynamics....
on it (equal to the person's weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
) and at the same time an equal and opposite ground reaction force is exerted by the ground on the person.
In the mentioned example, the ground reaction force coincides with the notion of normal force
Normal force
In mechanics, the normal force F_n\ is the component, perpendicular to the surface of contact, of the contact force exerted on an object by, for example, the surface of a floor or wall, preventing the object from penetrating the surface.The normal force is one of the components of the ground...
. However, in a more general case, the GRF will also have a component parallel to the ground, for example when the person is walking, which requires exchanging horizontal forces with the ground in order to achieve motion.
The use of the word reaction
Reaction (physics)
The third of Newton's laws of motion of classical mechanics states that forces always occur in pairs. Every action is accompanied by a reaction of equal magnitude but opposite direction. This principle is commonly known in the Latin language as actio et reactio. The attribution of which of the two...
derives from Newton's third law, which essentially states that if a force, called action, acts upon a body, then an equal and opposite force, called reaction, must act upon another body. The force exerted by the ground is commonly referred to as the reaction, although, since the distinction between action and reaction is completely arbitrary, the expression ground action would be, in principle, equally acceptable.
Ground Reaction Force is not equivalent to friction, however the normal component of the ground reaction force (GRF) can contribute to friction: the maximum force of friction between two surfaces equals the product of normal force, or the component of force applied perpendicular to the contact surface, and the coefficient of friction.