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Calculating total work using kinetic-work theorem

  1. Jul 25, 2013 #1
    1. A boy of mass m and a girl of mass 2m are initially at rest at the center of a frozen pond. They push each other so that she slides to the left at speed v across the frictionless ice surface and he slides to the right as shown above. What is the total work done by the children.



    2. KE=1/2(mv^2)



    3. I was thinking of adding the kinetic energies together. For the girl, I was thinking mv^2 and 1/2 mv^2 for the boy. This would give me 3/2(mv^2), but the correct answer should be 3mv^2. I need help understanding this concept, and how you would arrive to this conclusion. Thanks!
     
  2. jcsd
  3. Jul 25, 2013 #2

    Doc Al

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    Staff: Mentor

    You are given that the speed of the girl is v, so how her KE be mv^2?

    But first things first. How can you figure out the speed of the boy?
     
  4. Jul 25, 2013 #3
    By conservation of momentum! Thanks for pointing me in the right direction.
     
  5. Jul 25, 2013 #4

    Doc Al

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    But you were correct about that! (Her mass is 2m, not m. :redface:)

    Exactly!
     
  6. Jul 25, 2013 #5
    I had written her mass in terms of the boys mass. I was not clear on that in my initial post. Sorry about that. I should have used subscripts.
     
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