Tennis Ball

1. Dec 17, 2008

This is a really simple question. My math teacher was explaining how if you drop a ball with some amount of air in it, it will never stop moving because it will always bounce half of the height of its last bounce. Then my science teacher said the force of gravity would eventually stop the ball. Who is correct... (The math makes sense to me.. isn't the math already taking into account gravity)

2. Dec 17, 2008

mgb_phys

In theory it will never stop moving - in practice it will.
Similarly the rate that you coffee cools is proportional to the temperature difference between the coffee and the room. As the coffee approaches the room temperature the cooling rate slows and it will take an infinite time to cool down.

3. Dec 17, 2008

Gnosis

Perhaps it’s best to look at it this light.

Drop a ball from 4 feet. If each time it bounces back up halfway, by the time the ball bounces just 12 times, it’s already bouncing back up less than one thousandth of a foot. After its 22nd bounce, it’s bouncing back up less than one millionth of a foot. After the 32nd bounce, it's bouncing back up by less than one billionth of a foot. It won’t take very long before its movement is less than that of the orbit of an atom, so who’s going to be able to detect its movement as it continues? For all practical purposes, by the time you can no longer detect the ball’s bouncing motion, it’s essentially at rest. Movement of any sort by that time is considered “negligible”.

Hope this helps.