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Impulse/Momentum Problem

  1. Mar 30, 2009 #1
    1. The problem statement, all variables and given/known data
    1) A 0.1 kg ball is rolling across the floor at 0.5 m/s when it hits a wall and reverses direction without any change in speed.

    A) Can you determine the impulse delivered to the ball by the wall? If so, what is it? If not, explain why not.

    B) Can you determine the impulse delivered by the ball to the wall? If so, what is it? If not, explain why not.

    C) Can you determine the force exerted by the wall on the ball? If so, what is it? If not, explain why not.


    2. Relevant equations
    Jnet = [tex]\Delta[/tex]p
    p = m*v
    J = Fnet * [tex]\Delta[/tex]t


    3. The attempt at a solution
    My problem - I managed to calculate that the change in momentum for the ball is -0.1 m/s, but if this is the amount of impulse it exerts on the wall (since Jnet = Δp), wouldn't the wall exert an equal and opposite impulse? But wouldn't the wall's momentum and therefore change in momentum be zero, because Δp = m * Δv, and the wall never has any velocity.
     
  2. jcsd
  3. Mar 30, 2009 #2

    alphysicist

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    Homework Helper

    Hi adorrkable,

    Not to be picky, but remember the units are kg m/s.

    Actually that's the impulse the wall exerts on the ball. When you apply:

    [tex]
    \mbox{impulse} = \Delta p
    [/tex]

    the two sides of the equation have to be applied to the same system. But yes, the impulse from the wall on the ball is equal in magnitude and opposite in direction from the impulse of the ball on the wall.

    We wouldn't say that the no part of the wall never has any velocity, it's just that it's small enough to be ignored here. The wall will react in some way to the impact, but it's not important to the problem.
     
  4. Mar 30, 2009 #3
    So the ball exerts an impulse of -0.1 kg m/s, and the wall exerts an impulse of 0.1 kg m/s, am I correct?

    Thank you very much!
     
  5. Mar 30, 2009 #4

    alphysicist

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    Homework Helper

    If the ball's change in momentum is -0.1 kg m/s, then that is what happened to it; and so the impulse of -0.1 kg m/s is what was applied to the ball (and the negative of that is what the ball applied to the wall).

    That's what I meant about the equation; if you're looking at the ball as your system, then:

    (impulse applied to ball)=(change in ball's momentum)
     
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