If a penny falls on to a spinning DVD what happens?

Question

If a penny falls on to a spinning DVD what happens?

 

A DVD spins faster

B DVD spins slower

C DVD stops spinning

Answer 1

Hi Alex,

 

Perhaps the easiest way to look at this problem is from an energy perspective. If a DVD is spinning, it has some rotational kinetic energy. If you drop a penny onto it, will that rotational energy:

 

A) Increase

B) Decrease

C) Go away completely

 

A) doesn't really make sense, because to make it spin faster, you'd have to apply a force in the direction of the spin. Dropping a penny on the DVD wouldn't do that.

 

C) also doesn't make much sense, because the rotational energy can't just disappear completely, it has to be transferred somewhere, and unless the whole thing explodes, the entire amount of energy has nowhere to go.

 

B) makes sense, because part of the energy in the DVD is transferred into the penny, but not all of it. Since the DVD is larger, it has more than enough energy to do work on the penny to make it start spinning as well as continuing to spin itself, just at a slower speed.