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Homework Help: Tension Rope and Blocks Problem

  1. Apr 11, 2015 #1
    1. The problem statement, all variables and given/known data
    Two blocks, each with weight w, are held in place on a frictionless incline as shown in Figure. In terms of w and the angle a of the incline, calculate the tension in the rope connecting the blocks
    2. Relevant equations
    T = mg

    3. The attempt at a solution
    I cannot for figure out why the answer is Wsin(a)! I thought that since Tsin(a) = mg, T = w/sin(a)
  2. jcsd
  3. Apr 11, 2015 #2
    You have to look at the forces that are pulling along the surface. The force pulling downwards the slope is ##F_d=mgsin(a)##, the force pulling up is just the tension ##T##. For the block to be not moving along the slope these two have to be equal to eachother.
  4. Apr 11, 2015 #3


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    Staff: Mentor

    Hi DaveTan, Welcome to Physics Forums.

    Take a close look at the geometry of a block on a slope. The weight W = Mg forms the hypotenuse of the force triangle:

  5. Apr 12, 2015 #4
    Thanks! But shouldn't tension be pointing in the other direction? So towards the top-right instead of bottom left?
  6. Apr 12, 2015 #5


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    Science Advisor
    Homework Helper
    Gold Member

    Tension acts both ways (as does compression). The force it exerts on B will be up and to the right, that on A down and to the left.
    By the way, the reason your original approach went wrong is that the vertical forces include a component of the normal force.
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