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Elastic potential energy of two blocks of mass

  1. Oct 20, 2006 #1
    Two blocks of masses M and 3M are placed on a horizontal, frictionless surface. A light spring is attached to one of them, and the blocks are pushed together with the spring between them. A cord initially holding the blocks together is burned; after this, the block of mass 3M moves to the right with a speed of 1.50 m/s.
    (a) What is the speed of the block of mass M?
    (b) Find the original elastic potential energy in the spring if M = 0.350 kg.

    I already have the answer to a, which is obviously 4.5m/s. But, I am clueless as to part b. Please help!
     
  2. jcsd
  3. Oct 20, 2006 #2

    Hootenanny

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    What factors determine the potential energy stored in a spring and how do they relate to each other?
     
  4. Oct 20, 2006 #3
    It depends on how much is compresses or stretches, but that isn't known and as far as I know can't be calculated. It also depends on the stiffness of the spring but that isn't given either.
     
  5. Oct 20, 2006 #4

    Hootenanny

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    Okay, sorry a little misleading perhaps. What quantities must be conserved here?
     
  6. Oct 20, 2006 #5
    Momentum is conserved, and kinetic energy is conserved, so that means that potential energy must be conserved too. and change in kinetic energy = - change in potential energy..
     
    Last edited: Oct 20, 2006
  7. Oct 20, 2006 #6

    Hootenanny

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    Energy is conserved, it can however, be transformed from one form to another (i.e. from potential to kinetic).
     
  8. Oct 20, 2006 #7
    I think my problem was only calculating the kinetic energy of the smaller mass, because I thought if i calculated it for the system they would offset each other but they dont. So, KE=(.5)(.35)(.45^2) - (.5)(.35)(3)(1.5^2) = 2.3625 so potential energy is the opposite of that?
     
  9. Oct 20, 2006 #8

    Hootenanny

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    Note that kinetic energy is not a vector quantity, it is scalar. You should therefore add the individual kinetic energies to obtain the potential.
     
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