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A spring and two blocks

  1. Oct 12, 2009 #1
    1. The problem statement, all variables and given/known data

    Two blocks are pushed down on a spring with 75J of elastic potential energy. The spring is released and the blocks are allowed to fly freely (not sure I translated this sentence correctly, maybe separately instead of freely). Block A has a mass of 2 kg, block B has a mass of 3 kg. What is the highest velocity reached by these two blocks?

    2. Relevant equations
    Energy conversion
    Ei = Ef
    75 = .5*m1*(v1)^2+.5*m2*(v2)^2

    3. The attempt at a solution
    I figured out only one equation, I need two because I have two unknowns.

    75 = .5*2*(v1)^2+.5*3*(v2)^2
     
  2. jcsd
  3. Oct 13, 2009 #2

    Doc Al

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    Staff: Mentor

    Do you mean that a block is at each end of the spring and the two blocks are pushed together, thus compressing the spring?

    Hint: What else besides energy is conserved?
     
  4. Oct 13, 2009 #3
    D'oh! I should have realized that they were on opposite sides, thanks!

    Linear momentum is conserved (I think),
    0 = m1v1 + m2v2 = 2v1 + 3v2

    75 == .5*2*v1*v1 + .5*3*v2*v2
    0 == 2 v1 + 3 v2
    Gives v1 -> 6.71m/s, v2 -> 4.47m/s in opposite directions
     
  5. Oct 13, 2009 #4

    Doc Al

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    Staff: Mentor

    You got it.
     
  6. Oct 13, 2009 #5
    Thanks for the help. :smile:
     
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