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Bob (physics)
Encyclopedia
A bob is the weight on the end of a pendulum
most commonly, but not exclusively, found in pendulum clock
s.
, and minimises the length of pendulum required for a given period. Shorter pendulums allow the clock case to be made smaller, and also minimize the pendulum's air resistance. Since most of the energy loss in clocks is due to air friction of the pendulum, this allows clocks to run longer on a given power source.
Traditionally, a pendulum bob is a round flat disk, lens-shaped in section
, but bobs in older clocks often have decorative carving and shapes characteristic of the type of clock. They are usually made of a dense metal such as iron or brass
. Lead
is denser, but is usually avoided because of its softness, which would result in the bob being dented during its inevitable collisions with the inside of the clock case when the clock is moved.
In most pendulum clocks the rate is adjusted by moving the bob up or down on the pendulum rod. Moving it up shortens the pendulum, making it beat more quickly, and causing the clock to gain time. In the most common arrangement, the bob is attached to the pendulum with an adjustment nut at the bottom, on the threaded end of the pendulum rod. Turning the nut adjusts the height of the bob. But some bobs have levers or dials to adjust the height. In some precision clocks there is a smaller auxiliary weight on a threaded shaft to allow more fine adjustment. Tower clocks sometimes have a tray mounted on the pendulum rod, to which small weights can be added or removed, to adjust the rate without stopping the clock.
The weight of the bob does not itself affect the period of the pendulum. However, a heavier bob helps to keep the pendulum moving smoothly until it receives its next push from the clock's escapement
mechanism. That increases the pendulum's Q factor
, making the motion of the pendulum more independent of the escapement and the errors it introduces, leading to increased accuracy. On the other hand, the heavier the bob is the more energy must be supplied by the clock's power source and more friction and wear occurs in the clock's movement. Pendulum bobs in quality clocks are usually made as heavy as the clock's movement can drive. A common weight for the bob of a one second pendulum
, widely used in grandfather clocks and many others, is 15 lbs (6.8 kg).
Pendulum
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...
most commonly, but not exclusively, found in pendulum clock
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...
s.
Reason for use
Although a pendulum can theoretically be any shape, any rigid object swinging on a pivot, clock pendulums are usually made of a weight or bob attached to the bottom end of a rod, with the top attached to a pivot so it can swing. The advantage of this construction is that it positions the centre of mass close to the physical end of the pendulum, farthest from the pivot. This maximizes the moment of inertiaMoment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
, and minimises the length of pendulum required for a given period. Shorter pendulums allow the clock case to be made smaller, and also minimize the pendulum's air resistance. Since most of the energy loss in clocks is due to air friction of the pendulum, this allows clocks to run longer on a given power source.
Use in clocks
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
, but bobs in older clocks often have decorative carving and shapes characteristic of the type of clock. They are usually made of a dense metal such as iron or brass
Brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
. Lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
is denser, but is usually avoided because of its softness, which would result in the bob being dented during its inevitable collisions with the inside of the clock case when the clock is moved.
In most pendulum clocks the rate is adjusted by moving the bob up or down on the pendulum rod. Moving it up shortens the pendulum, making it beat more quickly, and causing the clock to gain time. In the most common arrangement, the bob is attached to the pendulum with an adjustment nut at the bottom, on the threaded end of the pendulum rod. Turning the nut adjusts the height of the bob. But some bobs have levers or dials to adjust the height. In some precision clocks there is a smaller auxiliary weight on a threaded shaft to allow more fine adjustment. Tower clocks sometimes have a tray mounted on the pendulum rod, to which small weights can be added or removed, to adjust the rate without stopping the clock.
The weight of the bob does not itself affect the period of the pendulum. However, a heavier bob helps to keep the pendulum moving smoothly until it receives its next push from the clock's escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...
mechanism. That increases the pendulum's Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
, making the motion of the pendulum more independent of the escapement and the errors it introduces, leading to increased accuracy. On the other hand, the heavier the bob is the more energy must be supplied by the clock's power source and more friction and wear occurs in the clock's movement. Pendulum bobs in quality clocks are usually made as heavy as the clock's movement can drive. A common weight for the bob of a one second pendulum
Seconds pendulum
A seconds pendulum is a pendulum whose period is precisely two seconds; one second for a swing in one direction and one second for the return swing, a frequency of 1/2 Hz....
, widely used in grandfather clocks and many others, is 15 lbs (6.8 kg).