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Homework Help: Stored spring energy

  1. Apr 22, 2010 #1
    Hi - Would someone check my method here please?
    Thank you

    1. The problem statement, all variables and given/known data

    Two trolleys A and B, of mass 0.70kg and 0.80kg respectively, are on a horizontal track and held together by a compressed spring. When the spring is released the trolleys separate freely and A moves to the left with an initial velocity of 4.0m/s.
    Calculate:
    a) the initial velocity of A
    b) the energy initially stored in the spring



    2. Relevant equations

    F=ma



    3. The attempt at a solution

    a) (0.70x4.0)=(0.80xV)
    v=(0.70x4.0)/(0.80)
    v=3.5m/s

    b) This is a transfer of elastic potential energy to kinetic energy -
    Energy=1/2mv^2
    mv=p(momentum)
    1/2(0.70x4.0)^2 + 1/2(0.80x3.5)^2
    =0.0392 + 3.92
    =3.9592 J
     
  2. jcsd
  3. Apr 22, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    This is good. You used conservation of momentum.

    Your mistake here was squaring mv, instead of just v.
    KE = 1/2mv^2, but you calculated 1/2(mv)^2. Not the same!
     
  4. Apr 22, 2010 #3
    ahh of course:
    silly boy!

    (1/2x0.7x4.0^2) + (1/2x0.8x3.5^2) = 10.5 J
     
  5. Apr 22, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Now you've got it. :smile:
     
  6. Apr 22, 2010 #5
    Thanks Doc
     
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