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Magnitude of Impulse help

  1. Nov 6, 2005 #1
    I'm getting very close, but not quite close enough on this problem:

    A baseball is struck by a bat such that it makes an angle of 41 degrees form the direction between the batter and the pitcher. The pitch was thrown at 37.995 m/s. The ball was hit with a speed of 6.258 m/s. What was the magnitude of the impulse in kg*m/s to two sig figs? The baseball has a mass of 0.147 kg.

    (I take it to mean the baseball leaves the bat 41 degrees off a line drawn from the pitcher to the batter.)
    1) I know that the impulse is equal to the change in momentum. So I said the change in momentum was: mballvball + (-vbat)mball = 4.6651
    2) I believed that that gave me the impulse of the ball if it went straight back so I said: 4.6651/cos41 to find the hypotenuse of that x comp I had.

    The answer is 6.3 kg*m/s and I get 6.2 kg*m/s.
    I did the same problem with different numbers and while their answer was 6.9 mine was 6.0. :frown: What am I doing wrong?
     
  2. jcsd
  3. Nov 6, 2005 #2

    Chi Meson

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    Is this supposed to be the speed of the ball after being hit by the bat? (Sounds like more of a "bunt")
     
  4. Nov 6, 2005 #3
    No. I take it as the speed at which the bat was swung. At the point when it hit the ball, it was going 6.258 m/s. (14 mph)
     
  5. Nov 6, 2005 #4

    Chi Meson

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    What did you find to be the speed of the ball after the collision with the bat?
     
  6. Nov 6, 2005 #5
    I didn't find the velocity. I don't see how I can without some kind of distance or time... should I be able to?
     
  7. Nov 6, 2005 #6

    Chi Meson

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    OK, here' my impression: the question is poorly worded. Without further information, I'm gonna have to insist that the 6.258 m/s is the speed of the ball. It's is an absurdly slow speed for either the ball OR bat, so I'm also thinking it was a typo.
    If this were the bat's speed, this would not be an easy problem AND it would require the mass of the bat.

    OK, so the impulse is the change in momentum which is (mass) times (change in velocity). Can you now find the 2-dimensional value for the vector change in velocity? (delta v)?
     
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