1. The problem statement, all variables and given/known data 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? 2. Relevant equations P=mV FΔt=ΔmV 3. 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?!