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Understanding Momentum and Impulse

  1. Feb 24, 2015 #1
    1. The problem statement, all variables and given/known data
    1) A hockey player makes a slap shot, exerting a force of 30.0N on the hockey pock for 0.16 seconds. What impulse is given to the puck?

    2) The hockey puck shot in exercise 1 has a mass of 0.115 kh and was at rest before the shot. With what speed does it head toward the goal.

    2. Relevant equations
    p=mv

    (F)triangle(t)= Triangle(p)

    3. The attempt at a solution
    1. If I'm using impulse formula correctly than multiplying 30N and 0.16 sec is 4.8

    2) I have a harder time understanding this, I'm not even sure if it's asking me to use the formula for momentum
     
  2. jcsd
  3. Feb 24, 2015 #2

    DrClaude

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    Units?

    What is the relation between momentum and speed?
     
  4. Feb 24, 2015 #3
    The solution can be found starting with with newtons classic:
    f = m * a
    Think about the definition for acceleration (a).
     
  5. Feb 24, 2015 #4

    haruspex

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    Yes, but given the first part of the question I would think the intent is to use momentum.
     
  6. Feb 25, 2015 #5
    Ok so how do I find the acceleration
     
  7. Feb 25, 2015 #6

    haruspex

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    You don't need to. See DrClaude's post.
     
  8. Feb 25, 2015 #7
    That's all the question says, the "at rest" part is kind of throwing me off
     
  9. Feb 25, 2015 #8

    haruspex

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    Why? Suppose it moves off at speed v. How much is the increase in speed? What is the increase in momentum?
     
  10. Feb 25, 2015 #9
    Force * time gives the value for impulse.
    The result equals the change in momentum, during the impulse.
    The original momentum is 0
     
  11. Feb 25, 2015 #10

    haruspex

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    Sonny18n already did that bit in part 1. DrClaude and I are trying to get Sonny18n to use that result together with the puck mass to solve part 2 directly rather than going back to the force given. I (we?) feel this is the intent of the question, and clearly something Sonny18n needs to learn how to do. The force * time approach is not always available, because it only works when the force is constant. Yes, it's constant here, but in other momentum questions it won't be.
     
  12. Feb 27, 2015 #11
    At rest means 0m/s right? So it won't affect the equation in a major way. But it's asking me to relate the answer from question 1 but it's not clear to me how impulse I can find the speed it takes.
     
  13. Feb 27, 2015 #12

    haruspex

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    Impulse is momentum. How do you normally assess the momentum of an object?
     
  14. Feb 27, 2015 #13
    Mass times velocity?
     
  15. Feb 27, 2015 #14

    haruspex

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    Yes.
     
  16. Feb 27, 2015 #15
    So the velocity is 0m/s then or..?
     
  17. Feb 27, 2015 #16

    haruspex

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    If the momentum is mass times velocity, what do you think a change in momentum for an object equals?
     
  18. Feb 27, 2015 #17
    But I don't have momentum. Only mass and no velocity
     
  19. Feb 27, 2015 #18

    haruspex

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    You have change in momentum and mass, and you want change in velocity.
     
  20. Feb 27, 2015 #19
    the first question is asking about impusle which the formula is ft=mv-mu. So in order to get the impulse just multiply the force given and the time. You may ask where does ft=mv-mu came from, it is derived from the F=ma. Acceleration equals to v-u/t. So when yiu substitute v-u/t into the acceleration , you will get F=mv-mu/t. The t is taken to the left side of the equation and it becomes Ft=mv-mu. mv-mu is the impulse and so ft is also impulse
     
  21. Feb 27, 2015 #20

    MacLaddy

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    Keep in mind that momentum is conserved, meaning that it can not change unless there is an outside influence acting on it- an impulse.

     
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