# Dropped tennis ball - please help?

After you drop a tennis ball of 0.5 kg from rest at a height of 3.8 m onto a hard floor, find the fallowing:

a) what will be the speed of the ball at the instant of contact with the floor?

v=8.63m/s

b) what force does the floor exert on the ball?

(f)=?N

c) Over what time does the force act in bringing the ball to rest?

(delta)t=?s

Thanks!

### 2 Answers

- simplicitusLv 71 decade agoFavorite Answer
You don't have enough information to solve the problem and the problem is as stated is clearly wrong - for example, the force exerted is not constant so the answer can't be a simple number.

What you can compute is the impulse:

http://en.wikipedia.org/wiki/Impulse

That will tell you the product of the stopping time and the average force.

For the stopping time, you might use an arbitrary quantity, such as the time it would take the tennis ball to drop half a radius.

Or, you can pretend that the tennis ball is a simple spring (though it isn't) and use the spring equations to get a total time.

Or, you can take another tack and assume that all the kinetic energy goes into compressing the air in the tennis ball adiabatically (with no heat loss) and with no energy loss. This (plus the dimensions of a standard tennis ball) lets you compute an approximation of how the force varies over time and how long it takes to stop (full compress) the ball. But even this requires data (such as the pressure inside the ball) which you don't have (but can approximate)

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