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Tension Rope and Blocks Problem

  • #1
9
0

Homework Statement


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
chemistry_8f635c0b85d41ce64d978925fca6415e.jpg

Homework Equations


T = mg

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)
 

Answers and Replies

  • #2
259
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.
 
  • #3
gneill
Mentor
20,799
2,778

Homework Statement


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
chemistry_8f635c0b85d41ce64d978925fca6415e.jpg

Homework Equations


T = mg

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)
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:

Fig1.gif
 
  • #4
9
0
Thanks! But shouldn't tension be pointing in the other direction? So towards the top-right instead of bottom left?
 
  • #5
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
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Thanks! But shouldn't tension be pointing in the other direction? So towards the top-right instead of bottom left?
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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