# Bouncing ball problem

1. Apr 29, 2005

### wootage

I have a problem that is multiple choice and I think that the choices may be wrong because the creator of the problem is know to have wrong answers. A 200g ball hits a wall perpendicularly with a velocity of 30 m/s and bounces back with a velocity of 20 m/s. The collison last for 5 ms, what is the average force exerted by the ball on the wall.

Seems like a straight forward problem that can be easily solved but I keep getting a different answer than what choices I have.

(.200 times 30 m/s) minus (.200 times 20 m/s) gives me 2 kg m/s.

so the force is 2 divided by the time which is 5ms
2/.005 is 400
so the answer should be 400 N ?
my choices are 6000 1000 4000 or 2000 N

2. Apr 29, 2005

### Staff: Mentor

Realize that velocity is a vector. Sign matters! If you call "towards the wall" as positive, then the initial velocity is +30 m/s and the final velocity is -20 m/s (away from the wall and thus negative). Does this change your answer?