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Homework Help: Newton's 2nd law problem

  1. Nov 19, 2012 #1
    1. The problem statement, all variables and given/known data

    http://www.diigo.com/item/image/2sb3i/ahcq [Broken]

    2. Relevant equations

    Ʃ[itex]\vec{F}[/itex] = m[itex]\vec{a}[/itex]

    3. The attempt at a solution

    For M :
    T - mg = ma
    T - 19.6 = 2a (1)

    For 2M :
    40 cos (150) - T = 4a
    from (1) : T = 2a + 19.6
    substituting :
    40 cos (150) - (2a +19.6) = 4a
    40 cos (150) - 19.6 = 6a

    a = [itex]\frac{40 cos (150) - 19.6}{6}[/itex] = -9 m/s^2

    but the right answer is 2.5 m/s^2.
    if I replace 150 with 30 the answer will be correct
    but it's 150 with respect to the positive x axis.
    what is the wrong please??
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Nov 19, 2012 #2
    What is direction of T for 2M?
     
  4. Nov 19, 2012 #3
    The sign of F cos 150 does not match what you've assumed in your equation.
     
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