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Homework Help: Energy (Potential, kinetic ) So stuck

  1. Oct 29, 2009 #1
    Energy (Potential, kinetic...) So stuck :(

    1. The problem statement, all variables and given/known data
    Two objects, m1 = 5.50 kg and m2 = 3.00 kg, are connected by a light string passing over a light frictionless pulley as shown in the figure below. The object of mass 5.50 kg is released from rest, h = 4.50 m above the ground.

    (a) Using the isolated system model, determine the speed of the 3.00 kg object just as the 5.50 kg object hits the ground. (I got 5.10 m/s)

    (b) Find the maximum height to which the 3.00 kg object rises.

    2. Relevant equations


    3. The attempt at a solution

    Part a was not a problem. It is part b that I'm stuck on. Common sense tells me that the maximum height that it will reach is equal to the height from which m2 is dropped. Apparently, my logic is faulty. So I tried the K1+U1=K2+U2 approach and am getting the wrong answer. I tried assuming that m2 had no potential energy when it reaches the bottom (and m1 would reach its maximum height), but wouldn't m2 also have no kinetic energy at the bottom, and m1 no kinetic energy at the top? And besides that, wouldn't their velocities be equal to zero, leaving me only with U1? I've literally been trying to figure this out for the last hour, what am I not seeing? Any help greatly appreciated!
  2. jcsd
  3. Oct 30, 2009 #2


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    Homework Helper

    Welcome to PF!

    Hi zomething! Welcome to PF! :smile:
    he he :biggrin:

    when m2 hits the ground, the rope will go slack (ie no tension), so m1 will just be a projectile, launched upward …

    find the speed of m1, and then use conservation of energy. :smile:

    (alternatively, regard m2 as continuing to have the same KE as it had just before it hit the ground)
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