# Homework Help: Need help.

1. Dec 6, 2004

### Rapta3

A tennis ball may leave the racket of a top player on the serve with a speed of 62.0 m/s. If the ball's mass is 0.0600 kg and it is in contact with the racket for 0.0200 s, what is the average force on the ball?

Would this force be large enough to lift a 60 kg person?
large enough or
not large enough

I got .0600x62=3.72

.0200/3.72=.00537

I dont think that is right because that seems way to small but I dont know what im missing...if it is incorrec that is.

2. Dec 6, 2004

### futb0l

W = mg so the force needed to lift a 60kg person will be ~ 600N.
a = 62/0.02 = 3100
F = ma = 0.06*3100 = 186

So it is not large enough.

3. Dec 6, 2004

### Galileo

The force on the ball equals the change of its momentum with respect to time.

$$F_{ave}=\frac{\Delta P}{\Delta t}$$

You already calculated its momentum when it leaves the racket, if we assume the ball is stationary before it is hit you can get the average force from the above formula.

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