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Pulley Problem / Coupled Motion Question

  1. Sep 28, 2007 #1
    I have a question about a question; I know how to solve it, but I want to know why I can't do something to get the answer:


    Problem is:

    Standard pulley problem setup. A mass, M, equaling 2.00 kg is on top of a table with a frictionless surface. A second mass, m, equaling 10.0 kg, is attached to it by a string and hanging off the table by a pulley. What will the tension in the string be if someone holds M at rest?

    Attempt:

    M= 2 kg
    m= 10 kg
    Ffriction= 0
    Ftension= ?

    Fxtotal= 0
    Fytotal=0

    Fxtotal= Ft - Ff
    why does 0 = Ft - 0 not work ?


    I know that would mean Ft would have to equal zero, and I know that's not the answer. Why is Fxtotal not zero? Or is the problem something else...

    Sum of Forces, x, = m*a, and acceleration is zero because the system is not moving, right? What's wrong with this logic...
     
    Last edited: Sep 28, 2007
  2. jcsd
  3. Sep 28, 2007 #2

    Chi Meson

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    Science Advisor
    Homework Helper

    You are leaving out the force of the hand holding block M.

    Net force is zero, in both x and y dimensions. There is no friction. The tension at one end of the string must balance the weight of the hanging mass. The tension at the other end of the string must the be balanced by the hand.
     
  4. Sep 28, 2007 #3
    Ohh yeahhhh. I knew it didn't make sense. Thanks a lot for clearing that up
     
  5. Sep 29, 2007 #4
    As m is at rest, the net force on it is zero. Hence, tension in the string = mg.
     
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