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Momentum of an object and kinetic energy

  1. Feb 11, 2007 #1
    1. The problem statement, all variables and given/known data
    the momentum of an object (originally with a non zero momentum) is doubled by doubling the speed. What happens to the kinetic energy? Justify your answer you should get a numerical answer.


    2. Relevant equations



    3. The attempt at a solution

    well i know that momentum is the product of mass and velocity and velocity has to do with speed but im not getting anywhere with this... i dont understand where a numerical value will come from someone please help me asap :frown:
     
  2. jcsd
  3. Feb 11, 2007 #2

    ranger

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    So you know that momentum increases proportionally with velocity. But do yo know the expression to find kinetic energy?
     
  4. Feb 11, 2007 #3
    is it 1/2mv^2 ?
     
  5. Feb 11, 2007 #4

    ranger

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    Yup. So can you take it from here?
     
  6. Feb 11, 2007 #5

    cepheid

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    The question you're supposed to answer is, "what happens to the kinetic energy?" When they say they want a numerical answer, they mean that they want you to answer in a quantitative way:

    "The kinetic energy increases by a factor of ____," where ____ is your numerical answer,

    and NOT in a purely qualitative way:

    "The kinetic energy gets bigger too."
     
  7. Feb 11, 2007 #6
    i still dont understand what numbers im using there is no numbers given....? kinetic energy is increasing? im sorry i feel stupid this is my first physics course though and my professor does not explian things well at all...
     
  8. Feb 11, 2007 #7

    ranger

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    The way you could do this is by taking an object with a particular mass, then have it move a some velocity. Once you have the momentum, then find KE. Then they want you to double the momentum. So its like doubling the velocity. Then take the new velocity and find KE. Then you can answer the question, what happens to KE. Makes sense?
     
  9. Feb 11, 2007 #8

    cepheid

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    I already answered this:

    So I've already given you part of the answer by stating that the KE increases. It's true that no numbers are given. You don't know what the starting KE is. But you know that it has some value. The key word in my explanation was the word "factor". If the speed doubles, you should be able to tell how the new value of the KE is related to the old one...ie by what factor it has changed (e.g. twice, three times...?). So in other words, the number you will obtain will give the value of the new KE relative to the value of the old KE (whatever that was). It won't give you the value in absolute terms. Does this difference between relative and absolute make sense?
     
  10. Feb 11, 2007 #9
    yes thank you both :)
     
  11. Jul 2, 2011 #10
    if the momentum doubles by doubling the speed (2mv= m*2v) then kinetic energy increases by 4 times...substitute (2v) for (v) in the formula for kinetic energy 1/2*m*v*v.....so the new KE is 1/2*m*2v*2v ....ie ....4(1/2*m*v*v)
     
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