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Homework Help: Momentum/Elastic Potential Energy

  1. Jan 8, 2009 #1
    1. The problem statement, all variables and given/known data

    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 as shown in the figure below. A cord initially holding the blocks together is burned; after that happens, the block of mass 3M moves to the right with a speed of 2.85 m/s.

    Image: http://www.webassign.net/serpse/serpse7_p9-4.gif


    a. What is the velocity of block mass M? Assume Right is positive and left is negative
    I got the solution to this. It was -8.55 m/s
    b. Find the system's original elastic potential energy, taking M= .300 Kg
    This one i don't understand how you can find the elastic potential energy without knowing the spring constant/distance compressed.
    2. Relevant equations
    Pinitial=Pfinal
    P=MV

    Us= 1/2 KX^2


    3. The attempt at a solution

    I got the solution for part a.
    m1v1+m2v2=0
    v1=-m2/m1 *v2
    v1=-3m/m*(2.85)
    v1=-8.55

    part b.

    potential energy for a spring is 1/2KX^2
    The problem gives the mass M as .300kg therefore mass 3M= .900kg
    1/2kx^2=1/2MV^2+1/2(3M)V^2

    I took a chance and i solved it and put it in and i was right!

    thank you physicznoob a lot!
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Jan 9, 2009 #2
    remember, period is dependant on the mass for a spring. for a pendulum it isnt.
     
  4. Jan 9, 2009 #3
    oh! thanks! i think i got it.
     
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