Bob (physics)

A bob is the weight on the end of a pendulum.

The use of a weight concentrated in a small, compact object enables the centre of gravity to be positioned close to the physical end of the pendulum, which minimises the length of pendulum required for a given period. To function efficiently, a bob must have minimal resistance to air (if the pendulum operates in air) to reduce the energy losses in the system. Traditionally, a pendulum bob is round, and lens-shaped in section, but bobs in older pendulum clocks often have decorative carving and shapes characteristic of the type of clock.

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.

The weight of the bob does not itself affect the period of the pendulum. However, a heavier bob (within reason) helps to keep the pendulum moving smoothly until it receives its next push from the clock's escapement mechanism. That is, it 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.

Origin of the term

The term 'bob' may have originated just after the pendulum clock was invented in 1657. Early verge clock pendulums were short and had very wide swings of 80° or more. They were often mounted on the back of clock cases, and when running the weight appeared to 'bob' out from behind the case on each side. These became known as "bob pendulums". [cite book|last=Britten|first=Frederick J.|title=Former Clock and Watchmakers and their Work|date=1894|publisher=E.& F.N. Spon|location=London|url=http://books.google.com/books?id=iHwCAAAAIAAJ&pg=PA170, p.170] Possibly the term spread to all pendulum weights.

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