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Homework Help: Conservation of energy of two blocks

  1. Mar 13, 2008 #1
    [SOLVED] conservation of energy

    1. The problem statement, all variables and given/known data
    Two blocks, of masses M = 1.9 kg and 2M are connected to a spring of spring constant k = 220 N/m that has one end fixed, as shown in the figure below. The horizontal surface and the pulley are frictionless, and the pulley has negligible mass. The blocks are released from rest with the spring relaxed.

    (a) What is the combined kinetic energy of the two blocks when the hanging block has fallen a distance of 0.090 m?

    (b) What is the kinetic energy of the hanging block when it has fallen that 0.090 m?

    (c) What maximum distance does the hanging block fall before momentarily stopping?
    m W0155-N.jpg

    2. Relevant equations


    ki + ui = kf + uf

    3. The attempt at a solution

    I solved A using u(grav) =kf +us and got 2.7 J
    For B i got 1.8 J

    C is where I have a problem. I know the kinetic energy must equal zero and somehow you find the height from u=mgh. I just can't seem to connect the two...
    Last edited by a moderator: Mar 13, 2008
  2. jcsd
  3. Mar 13, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The change in energy must be zero. You start with gravitational PE, which gets converted to spring PE.
  4. Mar 4, 2009 #3
    Re: [SOLVED] conservation of energy

    For part A, when you say you used Ugrav = final kinetic energy + Spring potential energy, could you please elaborate as to how that was accomplished? If final kinetic energy is 0.5*mass*velocity, how did you calculate velocity?

    For part B, I run into the same roadblock, the veocity.

    For C, I don't understand how the distance is 0. The answer is supposed to be 0.39m (I'm referring to the answsers in the back.

    I apologise for asking so much, but I would really like to understand how this problem is solved. Thank you in advance.

    EDIT: Just realized that the thread is about a year old!! I apologise for that...
    Last edited: Mar 4, 2009
  5. Mar 4, 2009 #4

    Doc Al

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    Staff: Mentor

    Re: [SOLVED] conservation of energy

    For both A and B you are solving for the final kinetic energy.

    The distance is not zero. (That earlier post of mine states that the change in energy is zero, not the distance. That's just a statement of energy conservation.)
    Yeah, it generally doesn't make sense to respond to an old post... but here we are!
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