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Homework Help: Bouncing ball problem

  1. Apr 29, 2005 #1
    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. jcsd
  3. Apr 29, 2005 #2

    Doc Al

    User Avatar

    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?
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