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Homework Help: Inclined Plane(s) Involving Forces

  1. Oct 3, 2009 #1
    1. The problem statement, all variables and given/known data
    Literally #19 from this link: http://physics.ucsd.edu/students/courses/summer2007/managed/physics2a/documents/chp6.pdf [Broken]

    The figure is not within the .pdf file, so I recreated the figure it references: http://img32.imageshack.us/img32/5701/figurephysics.png [Broken]

    2. Relevant equations
    [tex] F = ma [/tex]

    3. The attempt at a solution
    Working the problem on my own, I only am able to get to this step, where the tension forces cancel:

    [tex] m_ra + m_la = m_rg\sin(20) - m_lg\sin(60) [/tex]

    ... I am at a loss for the next step; particularly in the demonstrated solution, where they add and subtract the acceleration.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 4, 2009 #2


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    mr*a + ml*a = mr*g*sin(20) - ml*g*sin(60)
    Take all the mr terms to the left side:
    mr*a - mr*g*sin(20) = -ml*g*sin(60) - ml*a
    Too many minus signs so multiply everything by -1
    mr*g*sin(20) - mr*a = ml*g*sin(60) + ml*a
    Factor out the mr on the left side:
    mr[g*sin(20) - a] = ml*g*sin(60) + ml*a
    Divide both sides by the [ ] part
    mr = ml*g*sin(60) + ml*a all divided by [g*sin(20) - a]
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