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Homework Help: Two sliding masses problems

  1. Oct 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A block of mass m starts from rest from a height h and slides down along a loop with radius r. What should the initial height h be so that m pushes against the top of the track with a force equal to its weight? (ignore friction)

    2. Relevant equations
    U = mgh
    K = 1/2 mv^2
    F = mg = mv^2/r

    3. The attempt at a solution
    So the force should be equal to its weight (mg) which I set equal to mv^2/r. So at rest, m has a potential energy of mgh. At the bottom on the incline before entering the loop, m has a kinetic energy of 1/2 mv^2. Since there is no external forces, U = K. Solving the force equation, I got v^2 = gr. The height h = v^2/2g. My answer ends up being h = r/2, but I have the correct answer, 3r. What am I missing here?
    Last edited: Oct 9, 2008
  2. jcsd
  3. Oct 9, 2008 #2


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    Science Advisor
    Homework Helper
    Gold Member

    One question at a time, please (you'll get better responses that way).
    What force is this?
    The net force acting at the top of the loop is equal to mv^2/r. One of the forces acting is the normal pushing force on the block , given as mg. There is another force acting on the block also. You must include it in determining the net force.
    You should note that the net force at the top of the circle is more than just mg; and that the speed of the block at the bottom of the loop (bottom of incline) is not the same as its speed at the top of the loop. You might want to consider writing your U=K equation at the top of the loop, after drawing a sketch and identifying all the forces acting on the block at the top of the loop.
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