# Basic Forces Question

1. Oct 28, 2009

### jrbillbrian

1. The problem statement, all variables and given/known data

Pete serves a tennis ball of mass 54 grams at a speed of 53.6 meters per second. The ball is in contact with Pete's racket through a distance of 49.7 centimeters. What force does Pete apply to the ball?

54 grams = 0.054 kg
Speed = 53.6 m/s
49.7 cm = 0.0497 m

2. Relevant equations
Force = mass x acceleration
Vfinal = at + vi

3. The attempt at a solution
I am VERY lost, considering the fact that our teacher told us to try and figure it any way we could.

I tried this. 0.0497 m = 0.5(53.6 m/s) * t2
t = 0.0420183500 s

Then I thought of the statement
It takes 0.0420183500 seconds to travel 0.0497 m when traveling at 53.6 m/s

Then I tried Vfinal = a*t + Vinitial
56.3 m/s = a * 0.0420183500 s

a = 1339.890784 m/s2

Force = 0.054 kg * 1339.890784 m/s/s
Force = 72.4 N

Can anyone help me out with this please?

2. Oct 28, 2009

### JDHalfrack

Since you are given the distance over which the force was applied, you should probably use an equation that has distance.

The way I read this question, you are given the following:
$$\Delta$$x = .497m
vi = 0 (<-- not given, but I'm assuming the ball wasn't moving until he hit it)
vf = 53.6 m/s

You are trying to find acceleration.

Find an equation that has $$\Delta$$x, vi, vf, and a in it, and solve for a.

Once you find acceleration, you can use the F = ma equation to find the force.

Does this make sense?

3. Oct 28, 2009

### jrbillbrian

Yes, what you are suggesting seems to make sense. I will try it and let you know if I figure it out. Thanks

4. Oct 28, 2009

### jrbillbrian

Thanks! I got it. One major and obvious problem that I discovered was the fact that I converted it to 0.0497 m rather than 0.497. I ended up using Vf^2 = Vi^2 + 2a * delta x
I solved for 156 Newtons. Thanks

5. Oct 28, 2009

### JDHalfrack

That's the same answer I got too. Good job! Glad to help!