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Homework Help: Physics-Conservation of Momentum

  1. Sep 26, 2005 #1

    guit_666

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    Hey guys i can't seem to explain this question for my homework.

    "During a tennis serve, momentum gained by the ball is lost by a racket. If the momentum is conserved, why dosen't the racket's speed change much?

    Im not sure, but i think it might be because the ball is at rest and there is no force exerted on the racket, so the speed of the racket dosen't change.
     
    guit_666, Sep 26, 2005
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  3. Sep 26, 2005 #2

    Doc Al

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    Staff: Mentor

    Since the racket definitely exerts a force on the ball (right?), the ball must exert an equal and opposite force on the racket (Newton's 3rd law). So that's not the answer!

    What's the definition of momentum?
     
    Doc Al, Sep 26, 2005
  4. Sep 26, 2005 #3

    Pengwuino

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    Imagine yourself on an ice rink. What happens when you throw a ball vs. what happens when you "throw" (or more realistically, push) another person on the ice rink?.
     
    Pengwuino, Sep 26, 2005
  5. Sep 27, 2005 #4

    guit_666

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    hey guys i came up with a different solution,
    Is it correct that the velocity of the racket dosen't change because of perfect elastic collisions where the objects involes some change in shape and some energy is lost to heat, meanwhile the momentum and Kineticc Energy are conserved.
     
    guit_666, Sep 27, 2005
  6. Sep 27, 2005 #5

    Doc Al

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    But that has nothing to do with conservation of momentum. Instead, realize that conservation of momentum says that whatever momentum the ball gains, the racket loses.

    Again, what's the definition of momentum? What does it depend on? (If you had to calculate the momentum of the ball, what would you need to know?)
     
    Doc Al, Sep 27, 2005
  7. Sep 27, 2005 #6

    guit_666

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    the formula of momentum is p=mv so other then velocity,mass is needed, so is it because the mass of the 2 objects are both equal for the velocity of the racket to stay the same
     
    guit_666, Sep 27, 2005
  8. Sep 27, 2005 #7

    Doc Al

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    If the masses were the same, then both would have the same change in velocity. But the masses are not the same! One is a tennis ball, the other a racket. Hint: Which has the greater mass?

    Also: No one said the racket's speed remains the same, only that it doesn't change as much as the ball's speed does.
     
    Doc Al, Sep 27, 2005
  9. Sep 27, 2005 #8

    guit_666

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    So since the racket has a greater masss then the ball, therefore the momentum of the racket is greater then the ball, so if the momentum is greater the racket will still move in the same direction and the velocity will not change much.
     
    guit_666, Sep 27, 2005
  10. Sep 27, 2005 #9

    guit_666

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    Is that the right solution
     
    guit_666, Sep 27, 2005
  11. Sep 27, 2005 #10

    guit_666

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    So since the racket has a greater masss then the ball, therefore the momentum of the racket is greater then the ball, so if the momentum is greater the racket will still move in the same direction and the velocity will not change much.

    Is this the right solution
     
    guit_666, Sep 27, 2005
  12. Sep 27, 2005 #11

    Doc Al

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    No, not right. Once again, conservation of momentum says: The change in momentum of ball and racket are equal (but opposite). It says that right in the problem statement.
     
    Doc Al, Sep 27, 2005
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