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Homework Help: Dynamics - Blocks on an Incline Plane

  1. Dec 28, 2011 #1
    1. The problem statement, all variables and given/known data
    In the diagram below find the 1) acceleration of the system 2) T1 and T2
    The ramp is angled 35degrees to the horizontal and coeff of friction is 0.15

    2. Relevant equations

    The image at the top is the problem's figure.
    http://img155.imageshack.us/img155/4176/centralkootenayj2011122.jpg [Broken]

    3. The attempt at a solution

    Basically, I've redone this problem and I believe my answers are now correct. In my picture all necessary FBDs are drawn. Do I have the right idea?

    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Dec 28, 2011 #2


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    Your answers appear correct, but I have the following comments:

    1.) Drawing a Free Body Diagram of the system of 3 blocks, which you have done to calculate the acceleration of the system, is generally not a good idea for these particular type problems where motion of all blocks is not in the same direction, even though you did get the correct answer.
    2.) Are the 5kg blocks accelerating up the plane or down the plane?
  4. Dec 28, 2011 #3
    Fair enough, I see your point on the direction. As you point out, the blocks are going up while the 20kg block is going down, so what is correct for the acceleration? Would it be correct to say that the 20kg block accelerates at 4.26m/s2 [down], but the other blocks accelerate at 4.26m/s2 [up] ? I suppose however this would change the tensions would it not? ahhh...
  5. Dec 29, 2011 #4


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    You can consider such problems one-dimensional, saying that the motion happens along the rope and all objects move with the same acceleration. Decide the direction of the acceleration: this is the "forward" direction, and the forces act along the rope forward or backward.
    Write the equation of motion for each body, and add up all three: the tensions cancel and you get an equation with the acceleration of the system as variable. The hanging body will move downward, and those on the slope will slide upward. Your arrow points in the wrong direction.
    I suggest to type in the solution, I can not follow your handwriting.

  6. Dec 29, 2011 #5


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    yes, correct
    No, you have the correct tensions, because you assumed the correct direction of motion in your FBD's, even though your arrow shows otherwise.
  7. Dec 29, 2011 #6
    Oh, I now see the arrow you're referring to. That was a silly error, due to the net force the 20kg block would be successful in accelerating downwards. Sorry about that arrow.

    Thanks though guys!
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