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Homework Help: Tension/pulley problem, Newtons second law

  1. Sep 23, 2010 #1
    1. The problem statement, all variables and given/known data
    Two objects A and B, with masses m1=1.00kg and m2=2.00kg, are connected with an ideal string. Object A is moving on an ideal incline with 0 friction. Object B is moving vertically. A force F is working on object B. Its magnitude is 6.00N. Object b is accelerating downward, with a magnitude og 5.50 m/s^2.

    a) Draw a free body diagram for the two objects
    b) Find the tension force between the objects
    c) Find the angle beta

    2. Relevant equations
    Newtons second law

    3. The attempt at a solution
    Made a free body diagram for each object. Then applied Newtons second law in the x-direction of object A and in the y-direction og object B.
    sigma F_x=F-T-m1*sin(beta)=m1*a
    sigma F_y=n-m1*g*cos(beta)
    sigma F_y=f+T-m2*g=ma

    Solved this for the variable T, and ended up with t=20.6 N. Obviously I am doing something wrong here, because when I try to find the angle beta, I end up with cos(beta)>1.

    I am not sure whether the force F should be in the free body diagram og object A at all.

    Thanks in advance.
  2. jcsd
  3. Sep 23, 2010 #2
    Forgot to upload this earlier. :)

    Attached Files:

  4. Sep 23, 2010 #3


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    Gold Member

    Does F show up in this free body diagram? And since the block is accelerating down the plane, you have your signage wrong. And you forgot to include the 'g' for the weight component down the plane.
    = ? (no acceleration perpendicular to the plane, but this equation is not needed in this problem)
    signage error....net force is down....choose down direction as positive.
    That is right, get it out of there...
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