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Homework Help: Finding impulse

  1. Jun 18, 2009 #1
    1. The problem statement, all variables and given/known data
    a 0.144kg baseball is pitched horizontally at +38m/s. after it is hit by a bat, the baseball moves horizontally at -38m/s.

    a) what impulse did the bat deliver to the ball

    b) if the bat and the ball were in contact for 8.0*10^-4 seconds, what was the average force that the bat exerted on the ball?

    c) find the average acceleration of the ball during its contact with the bat.

    2. Relevant equations
    J = Ft
    Ft = mv1 - mv2

    3. The attempt at a solution

    a) Ft = mv Ft = 0.144 * -38 Ft = -5.472 kg m/s (they got -11 kg m/s)

    b) F(.0008) = 5.472
    F = 5.472/.0008
    F = 6840 N (they got 14000 N)

    c) F = ma 6840 = 0.144a a = 6840/0.144 a = 47500m/s^2 (they got 97000m/s^2)

    i know i am doing something seriously wrong here but i have no idea what??
  2. jcsd
  3. Jun 18, 2009 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Well, your part (a) doesn't seem to make a lot of sense. You didn't use any of the "relevant equations" that you listed. You said the impulse was equal to the change in momentum:

    J = p2 - p1 = mv2 - mv1

    So why not calculate that? What is the final momentum? What is the initial momentum? What is the difference between the two?

    Your part (a) only calculates the final momentum, and multiplies it by Ft for no discernible reason.
  4. Jun 18, 2009 #3
    J ≡ ΔP
    ΔP ≡ Pf - Pi

    Your attempt at a solution makes no sense. Try using the initial momentum and final momentum to find the impulse.

    Then see how the first equation is relevant to the average force.
    J = Faverage*Δt
  5. Jun 18, 2009 #4
    wow. yeah i dont know what i was doing before
    i understand it now. serious brain malfunction.

    thanks for the help people
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