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Homework Help: Solve for the amount of force on an 8kg ball

  1. Jan 22, 2009 #1
    1. The problem statement, all variables and given/known data
    "solve for the amount of force on an 8kg ball rolling at 2m/s when it bumps into a pillow and stops in 0.5 seconds."
    ...and...
    "how much force does the pillow exert on the ball?"


    2. Relevant equations
    net momentum before = net momentum after?
    Ft=(change in) mv?


    3. The attempt at a solution
    8kg x 2m/s = 16N of force.....or 16 kg.m/s

    but whats the stopping got to do with it?
    isnt both halves of this question the same?
    Im so confused :S

    Please help
     
  2. jcsd
  3. Jan 22, 2009 #2

    LowlyPion

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    Re: Momentum

    F = ma = m*Δv/Δt
     
  4. Jan 25, 2009 #3
    Re: Momentum

    ....That doesn't help me...

    Sorry, I'm new to physics and don't understand a lot of it
     
  5. Jan 25, 2009 #4

    LowlyPion

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    Re: Momentum

    Δ stands for change in something.

    Δp is the change in momentum. You can also write that as m*Δv.

    So initial momentum is 8 kg* 2 m/s.
    Final p is 0. Δp = 16 then right?

    The change in time Δt is just the .5 sec.

    So F = Δp/Δt = 16 / .5 = ... ?
     
  6. Jan 25, 2009 #5
    Re: Momentum

    Ahhhhhh, I see.
    32.



    But what force does the pillow exert back?
    Isn't it equal and opposite? But wouldn't that cancel?

    Would it be 8N, but with no momentum?
     
  7. Jan 26, 2009 #6

    LowlyPion

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    Re: Momentum

    32 N is the force on the ball to stop it.

    By the statement of the problem it was from the pillow wasn't it?
     
  8. Jan 26, 2009 #7
    Re: Momentum

    Yeah, the pillow was the reason for the ball stopping...
     
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