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Homework Help: Help with a tension problem

  1. Feb 18, 2007 #1
    hi i cannot seem to figure this problem out at all. any help is appreciated.

    A 30 kg block (M1) is placed on a frictionless plane that inclines at a 30° angle with respect to the surface of Earth. This block is connected to another 20 kg block (M2) via a weightless rope over a frictionless, ideal pulley. The second block is hanging vertically, as shown in the figure. What is the tension of the rope?


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  2. jcsd
  3. Feb 18, 2007 #2


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    What are your thoughts on the question? If a mass isn't moving, what can you say about the resultant force on it?
  4. Feb 18, 2007 #3
    i believe that if the mass isn't moving, its net force is 0 because there is no acceleration. I'm just not sure what to use as my equations. this type of problem has always confused me for whatever reason.
  5. Feb 18, 2007 #4
    draw a force diagram for each blocks to see what forces are acting on them. As you said, the sum of these forces must be 0 because the blocks aren't moving.
  6. Feb 18, 2007 #5
    ok well on mass 1 i got the normal force going up, mgcos30 going down, tension going to the right and mgsin30 going left. on mass 2 i got tension up and mg down. so this would mean that T=mgsin30 or T=mg, but those are not right. theres something else that i am not seeing
  7. Feb 18, 2007 #6
    yeah actually i don't see how the blocks aren't moving because they should with those given quantities. problem was a little ambiguous on whether or not the blocks were moving. instead of making the sum of the forces equal to 0, make it equal to what it should be when the blocks are moving, which is mass times the block's acceleration.
  8. Feb 18, 2007 #7
    My advice is to assume acceleration. Let us say that M1 is sliding down the incline plane, so write your equation based off of that. If it turns out that M1 is going up, it shouldn't really matter.
  9. Feb 19, 2007 #8
    ok would the equation be ma+mgsin30=T? i'm not entirely sure. and then how do i know what the acceleration is? thanks
  10. Feb 20, 2007 #9
    yes , I think your equation is right.
    i want ask futher question , is "a" and "g" in the same direction?
    I think M2g+M1gsin30=T would be better.

    or I think in a wrong way?
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