If both friction and air resistance were eliminated from acting on the swinging pendulum, would gravity act on the pendulum to slow it down and eventually stop?

Asked by: John Barrus

Answer

No, it wouldn't. If all friction and air resistance was eliminated (plus, the losses due to deformation of the string and the like) the pendulum, under ONLY the effect of gravity, would keep swinging indefinitely.

That's because gravitation is a 'conservative' force, it does not drain any energy from the object moving under it effect, it just converts the energy from one form to another. When the pendulum reaches either end at its highest point, all the energy is potential energy, and the kinetic energy is zero. At the bottom of the swing, the kinetic energy is maximum, while potential energy is minimized. No energy is transferred out of the system, so it must keep moving. In the presence of friction however, energy is removed from the system in the form of heat. (The air heats up a little, the contact point of the string heats up a little... etc).
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey