Magnitude of average force

  • Thread starter Crichar1
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  • #1
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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

5.5+3.75=9.25m/s
 

Answers and Replies

  • #2
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What do you know about mechanics that will help? Any equations?

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

The Bob
 
  • #6
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mass x velocity

44kg x 8m/s= 352

30kg x 8m/s=240

352-240=112m/s

didn't seem right but i thought i would try it anyways
 
  • #7
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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
 
  • #8
49
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i tried the linear momentum equation..mass * velocity and the answer was still incorrect
Conservation of momentum. The total momentum before she jumps on the board=total momentum after she jumps on the board.
 

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