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Collision and Impulse, Linear Momentum

  1. Jul 18, 2006 #1
    can someone help? I have no isea how to do this problem.

    A force in the negative direction of an x axis is applied for 21 ms to a 0.40 kg ball initially moving at 10 m/s in the positive direction of the axis. The force varies in magnitude, and the impulse has magnitude 32.4 Ns.

    (a) What is the ball's speed just after the force is applied? m/s

    (b) What is the average magnitude of the force? N
     
  2. jcsd
  3. Jul 18, 2006 #2

    Doc Al

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    Staff: Mentor

    Tell us what you know about impulse, momentum, and the impulse-momentum theorem.
     
  4. Jul 18, 2006 #3
    i know about elastic and inelastic collsions
     
  5. Jul 18, 2006 #4
    i know the formula for those
     
  6. Jul 18, 2006 #5
    i know p=mv
     
  7. Jul 18, 2006 #6

    Doc Al

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    They gave you the impulse. What does that tell you?
     
  8. Jul 18, 2006 #7
    p1i+p2i=p1f+p2f
     
  9. Jul 18, 2006 #8
    i should use p=mv

    but thsat would give be AN INTIAL SPEED OF 81

    32.4=.4v
     
  10. Jul 18, 2006 #9

    Doc Al

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    impulse-momentum theorem

    Impulse = [itex]\vec{F}_{ave} \Delta t = \Delta \vec{p} = m \Delta \vec{v}[/itex]
     
  11. Jul 18, 2006 #10
    that gives me 91

    32.4=.4(v-10)
     
  12. Jul 18, 2006 #11

    Doc Al

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    What's the direction (and thus sign) of the impulse compared to the direction of the initial velocity?
     
  13. Jul 18, 2006 #12
    negative to the x axis then postivite the force is postive
     
  14. Jul 18, 2006 #13

    Doc Al

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    Answer these questions:
    What's the direction of the initial velocity?
    What's the direction of the impulse?
    What's the direction of the change in velocity?
    What's the direction of the final velocity?
     
  15. Jul 18, 2006 #14
    the direction of the intial volcity is positve.
    the direction of the impulse is negative
    the direction of the change in velocity is negative

    the direction of the final velocity is negative
     
  16. Jul 18, 2006 #15

    Doc Al

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    Good. So you should have no problems calculating the final speed.
     
  17. Jul 18, 2006 #16
    ok thank you
     
  18. Jul 20, 2006 #17
    After my analysis, first get the initial momentum of the ball in the positive x-direction. Second, get the force applied on the ball in the negative x-direction. Impulse is mass x change in velocity. Problem is whether the total force acting on the ball is sufficient to change the direction of ball.
     
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