# Homework Help: Newton's Second Law finding value of force problem

1. May 28, 2012

### goomer

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

If a soccer ball of mass 0.5 kg is kicked with a speed of 14 m/s and hits a rock and is in contact with it for 0.18 seconds, with what force will it hit?

2. Relevant equations

Fnet = ma

3. The attempt at a solution

I have no idea what to do to get the acceleration for the above formula. It can't possibly be just 14/0.18 can it?

2. May 28, 2012

### Infinitum

Force can also be defined in terms of momentum and time. What is it, and can you use that in this problem?

3. May 28, 2012

### goomer

Oh I see, so:

Force = Momentum/Time

Momentum = Mass * Velocity = 0.5 kg * 14 m/s = 7

Force = 7/0.18 s = 38.9N right?

4. May 28, 2012

### Infinitum

Yep!

Force is actually the change in momentum per unit time. But since the ball was in contact with the stone for that time, and its velocity was zero then, your equation worked out.

5. May 28, 2012

### goomer

Thank you so much for your help!