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Homework Help: Help calculating impulse

  1. Mar 3, 2010 #1
    A 0.300 kg baseball just before and after it collides with a bat. Just before, the ball has a velocity v1 of magnitude 12.0 m/s and angle θ1 = 35°. Just after, it is traveling directly upward with velocity v2 of magnitude 10.0 m/s. The duration of the collision is 1.5 ms.
    (a) What is the magnitude of the impulse on the ball from the bat?

    I tried solving a by

    pi = (.3)(12)(cos215i+sin215j) = -2.95i + 2.06j
    pf = (.3)(12)j = 3.6j
    i = pf - pi = 3j - (-2.95i - 2.06j) = 2.95i +5.66j

    mag = sqrt(2.95²+5.66²) = 6.38

    what am i doing wrong?
     

    Attached Files:

  2. jcsd
  3. Mar 3, 2010 #2

    berkeman

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    Staff: Mentor

    Would the delta-time be part of the relevant equations....?
     
  4. Mar 3, 2010 #3
    pf should be 10 and not 12 right? shoot, how could i miss that, just want to be sure because i just have one more attempt

    pi = (.3)(12)(cos215i+sin215j) = -2.95i + 2.06j
    pf = (.3)(10)j = 3j
    i = pf - pi = 3j - (-2.95i - 2.06j) = 2.95i +5.06j

    mag = sqrt(2.95²+5.06²) = 5.86


    correct?
     
  5. Mar 3, 2010 #4

    berkeman

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    Staff: Mentor

    You didn't answer my question. Where should the 1.5ms enter into your calculation of the impulse?
     
  6. Mar 3, 2010 #5
    i dont think it shouldnt enter because

    p = mass vs velocity

    pf = 0.3 x 10 = 3j
    pi = .3 x 12(cos215i+sin215j) = -2.95i + 2.06j

    right?
     
  7. Mar 3, 2010 #6

    berkeman

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    Staff: Mentor

    My apologies. I was misremembering the definition of Impulse.

    It looks like you just have a sign error in the y component of the incoming ball's momentum:

    The diagram in your original post (OP) shows the ball coming in at a downward angle.
     
  8. Mar 3, 2010 #7
    yes you are correct but anyways its not going to affect the answer since we take the absolute value of it, but something and i think this is correct

    pi = (.3)(12)(cos215i+sin215j) = -2.95i + 2.06j
    pf = (.3)(10)j = 3j
    i = pf - pi = 3j - (-2.95i - 2.06j) = 2.95i +5.06j

    mag = sqrt(2.95²+5.06²) = 5.86


    but just want to verify since its my last attempt to the question
     
  9. Mar 3, 2010 #8

    berkeman

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    Staff: Mentor

    You do not take any absolute values or do sqrt(squares) magnitude calculation until after you have subtracted the vectors. So the direction of the y component of the incoming baseball (negative) is important, since the final direction is upward (positive).
     
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