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## Homework Statement

A tennis ball (m 0.2 kg) is thrown at a brick wall. It is

traveling horizontally at 12 m/s just before hitting the

wall and rebounds from the wall at 8 m/s, still traveling

horizontally. The ball is in contact with the wall for 0.04 s.

What is the magnitude of the average force of the wall on

the ball?

## Homework Equations

P=mV

FΔt=ΔmV

## The Attempt at a Solution

I didn't even know where to begin to I went to the solutions manual which did this:

[-(0.2kg)(8ms)-(0.2kg)(12ms)] / 0.04s = -100N; magnitude would be 100

Where the crap is the negative sign coming from?!