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Homework Help: Conceptual question on momentum

  1. Jul 21, 2010 #1
    1. The problem statement, all variables and given/known data
    When a sprinter bursts from the starting blocks, he/she generates considerable forward momentum. Is this an example where momentum is not conserved?

    2. Relevant equations

    3. The attempt at a solution
    I say yes, because there is no interaction of two systems. The sprinter just pushes off and doesnt act on another object.

    Am I correct? Can someone help me elaborate this?

  2. jcsd
  3. Jul 21, 2010 #2
    well, if sprinter stands still at first and then gains some speed, then there is a change in momentum. To produce the change in momentum of some body (sprinter in this case) force is needed. But forces always comes in pairs - if some object is pushing sprinter, sprinter must push this object with the same force but in opposite direction (thats Newton's third law). What could this object be? If i'm correct, this object is very very large and massive...
  4. Jul 21, 2010 #3
    No. The sprinter just pushes off the starting blocks and hence act on another object which is the starting blocks . Start by defining what is included in the system. Earth, starting blocks, athlete, ..
  5. Jul 22, 2010 #4
    Let's see if i got this. The system includes the Earth. The sprinter pushes off and exerts a force down on the earth and therefore generates the forward momentum.

    is that a start?
  6. Jul 22, 2010 #5
    If the sprinter gains forward momentum, and momentum is always conserved, (no overall gain or loss) then something somewhere else must...?
  7. Jul 22, 2010 #6
    does it have to do with newton's third law? Something else is pushing against the sprinter?

    I'm not good at concepts, and i know it's important to understand in order to do the questions.

    please help me understand...THANKS
  8. Jul 22, 2010 #7
    The questioner is testing your confidence.
    He/she is inviting you to make a mistake by handing you the suggestion that 'this is a case where momentum is not conserved'.

    What you have to do is to figure out how the sprinter can gain momentum. He has to get it at the expense of some other object.

    What would happen if the sprinters blocks were not fastened to the ground properly?
  9. Jul 22, 2010 #8
    The sprinter is gaining momentum from pushing off the block. If the blocks weren't fastened then he won't be able to push off.
  10. Jul 22, 2010 #9
    And what would happen to the blocks?
  11. Jul 22, 2010 #10
    He gains forward momentum when he pushes off from the blocks because he exerts force against it and that generates the momentum.

    The blocks would slip.
  12. Jul 22, 2010 #11
    If momentum is conserved, how can it be 'generated'? It has to come from somewhere or be balanced by a momentum in the opposite direction.

    Keep thinking about the blocks. If the sprinter were in outer space when he pushed off from the blocks what would happen to the blocks?
    What would happen if he used BIG blocks.
    What would happen if he used some REALLY, REALLY HUGE blocks?
    What if he had blocks as big as the earth?
  13. Jul 23, 2010 #12
    The blocks would push back against the sprinter. According to Newton's third law, there is a reaction to an action.

    The bigger the blocks, the more force is exerted.

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