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Homework Help: Work, energy and power on a slope.

  1. Apr 27, 2009 #1
    1. The problem statement, all variables and given/known data
    The diagram shows a particle A of mass m which can move on the rough surface of a plane inclined at an angle theta to horizontal ground, where theta = arcsin 0.6. A second particle B of mass 2m hangs freely attached to a light inextensible string which passes over a smooth pulley fixed at D. The other end of the string is attached to A. The coefficient of friction between A and the plane is 1/4. B is initially hanging 3m above the ground and A is 2m from D. When the system is released from rest with the string taut A moves up the plane.
    (a) Find the initial acceleration of A.
    When B has descended 1m, the string breaks. By using the principle of conservation of energy.
    (b) calculate the total distance moved by A before it first comes to rest.

    http://img135.imageshack.us/img135/9503/scan0001small.gif [Broken]

    2. Relevant equations
    K.e. = 1/2mv2

    G.p.e = mgh

    3. The attempt at a solution
    a) 2mg - T = 2ma
    T - cos36.87 = ma
    2mg - 0.8mg = 3ma
    1.2g = 3a
    a = 3.92ms-2

    b) mgh = 2mg
    2(0.6) = 1.2

    I got part a right but b is meant to be 1.5m. Not sure why I ain't getting the right answer...
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 27, 2009 #2


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    Homework Helper

    For b) determine the kinetic energy of A at the moment the string breaks.

    The work of friction .8m*g*X = 1/2m*v2 is what will slow it to a stop.

    X is the additional distance past 1m that you are looking for.
    Last edited: Apr 27, 2009
  4. Apr 27, 2009 #3
    Wow, thanks again for your help! Got the right answer now! Thank you!
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