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Homework Help: AP Physics C Mechanics: Conservation of Energy Problem

  1. Sep 30, 2009 #1
    1. The problem statement, all variables and given/known data
    A block of mass m hangs on the end of a cord and is connected to a block of mass M by a pulley arrangement. (m hangs freely, M is attached to the string, and is on a table top. (So m is attached to a string which goes therough a pulley changing the strings direction 90 degrees where it goes through a pulley attached to M and back to a block on the table. So, for every unit m moves M moves two, sorry this is hard to explain without a diagram.) Using energy considerations,
    a.) find an expression for the speed m as a function of the distance fallen,
    b.) repeat the previous assuming that a friction of µ acts on the block on the table. Assume everything is at rest.

    I dont think it is pertinant, but, it has a few latter parts which are calculus based that I was able to get using what I know to be the answer to a and b.

    2. Relevant equations
    F=µN (N=mg)
    MAybe some kinematics equations for part B?)

    3. The attempt at a solution
    I think I got part a, just a math error.

    Using the formula K(m)+K(M)+U2(m)=U(m) U2 being the new potential energy after a change in distance, and then
    1/2mv^2+1/2m(v/2)^2(It will be half the speed of m)+mgh2=mgh
    and solving for M, I think I am getting the correct answer

    For B, that is including coefficient of friction I think I am having a lot more trouble. My first attempt was to attempt to factor in the friction into the velocity, but that got an incorrect answer. I then tried tog et force, and integrate (integral of NetForce dx=∆K)

    Can anyone point me in the right direction?
  2. jcsd
  3. Sep 30, 2009 #2


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    Homework Helper

    I don't see how you can find the acceleration using the energy approach.
    I think you will have to work with forces. The force of gravity on m is what makes the masses accelerate. Can you write an F = ma equation for each of the masses? Would the force pulling M be half the force with which m pulls on the rope?
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