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Homework Help: Magnitude of average force

  1. Mar 1, 2009 #1
    A woman with a mass of 44 kg runs at a speed of 8 m/s and jumps onto a giant 30 kg skateboard initially at rest. What is the combined speed of the woman and the skateboard?

    44 kg / 8 m/s=5.5
    30 kg / 8m/s=3.75

  2. jcsd
  3. Mar 1, 2009 #2
    What do you know about mechanics that will help? Any equations?

    The Bob
  4. Mar 1, 2009 #3
    Think momentum.
  5. Mar 1, 2009 #4
    i tried the linear momentum equation..mass * velocity and the answer was still incorrect
  6. Mar 1, 2009 #5
    But what did you do to get this incorrect answer?

    The Bob
  7. Mar 1, 2009 #6
    mass x velocity

    44kg x 8m/s= 352

    30kg x 8m/s=240


    didn't seem right but i thought i would try it anyways
  8. Mar 1, 2009 #7
    Right, now I see what you're doing. Have you heard of something called 'conservation of momentum'? Basically, the momentum before a 'change' must be equal to the combined momentum afterwards. You've, therefore, wrongly assumed that the skateboard has velocity of 8ms-1.

    So... try imagining the woman and the skateboard as one mass after they have 'combined'.

    The Bob
  9. Mar 1, 2009 #8
    Conservation of momentum. The total momentum before she jumps on the board=total momentum after she jumps on the board.
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