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Mechanics Question

  1. Nov 6, 2005 #1
    Hey everyone, I'm having trouble on a homework problem and hopefully someone can help me.

    A tennis player receives a shot with the ball (0.0600 kg) traveling horizontally at 58.0 m/s and returns the shot with the ball traveling horizontally at 38.0 m/s in the opposite direction. (Assume the initial direction of the ball is in the -x direction.)

    (a) What is the impulse delivered to the ball by the racquet?

    (b) What work does the racquet do on the ball?


    I was able to get (a), but (b) is stumping me. I know momentum is conserved, but they don't provide any mass information on the racquet so I'm confused. Any help is appreciated.
     
  2. jcsd
  3. Nov 6, 2005 #2
    The work done on an object is given by a change in kinetic energy.
     
  4. Nov 6, 2005 #3

    lightgrav

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    The momentum of the ball CHANGED due to the Impulse by the racquet.

    The KE of the ball was CHANGED by the Work done by the racquet.
    (note that Work done on an object might change its PE instead)
     
  5. Nov 6, 2005 #4
    Ok, I got it. Thanks for the help.
     
  6. Nov 6, 2005 #5
    Follow-up question, this one is in two-dimensions so I'm kind of confused.

    In a slow-pitch softball game, a 0.200 kg softball crossed the plate at 12.00 m/s at an angle of 40.0° below the horizontal. The batter hits the ball toward center field, giving it a velocity of 50.0 m/s at 30.0° above the horizontal.

    (a) Determine the impulse delivered to the ball.
    (b) ? ° (above the horizontal)

    I tried doing: (-12cos40*.2) + P(initial bat) = (50cos30*.2) + P(final bat)
    Then I solved for "P(final bat) - P(initial bat)" but its saying this is the incorrect answer. Please help.
     
  7. Nov 6, 2005 #6

    lightgrav

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    You don't know enoughabout the Forces exerted on the bat
    to claim that it is an isolated object.

    You can write about the ball alone, however:

    p_ball,initial,x + Sum (F_on_ball,x) Dt = p_ball,final,x
    p_ball,initial,y + Sum (F_on_ball,y) Dt = p_ball,final,y

    keep x and y components separate, and watch +/- signs.
     
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