How can I explain how air resistance damps a pendulum's motion

1. Jan 29, 2013

JamieGreggary

Basically I have an experiment which involves attaching circular discs of varying radius to a pendulum, monitoring how its amplitude varies with time.

In my write up I need to include an explanation of how air resistance causes a damped oscillation over time. So far I have written:

Air resistance works by the bombardment of gas particles on the circular disc. These collisions cause a change in the gas particles momentum, and hence a force exerted in the opposite direction to the motion of the circular disc. As a result of this, the velocity of the pendulum decreases and energy in the system is lost. This loss of energy means that the pendulum will have insufficient energy to reach the same amplitude as on its last peak, and as a result the amplitude diminishes over time.

I'm not too sure if this explanation is correct (see below of alternative idea). It is also incredibly brief, and am struggling to find inspiration on how else I could explain it/other ideas to include.

Due to the conservation of momentum, when the moving pendulum collides with the almost stationary particles, energy is transferred from the pendulum and onto the particle. This results in the loss of the pendulums momentum and velocity, and an increase in the particles momentum and velocity.

^ Then I'd add the last sentence of the first idea to this - but again, I'm not sure which of the explanations is correct (if any)

Thanks for any help guys!

Last edited: Jan 29, 2013
2. Jan 29, 2013

haruspex

It makes more sense to me to deal in terms of energy rather than momentum here. Clearly both are true, but the question which could be asked is why the process only works in the one direction. If you want to cover that you'd need to go into a discussion of entropy.