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Homework Help: Mechanics Help

  1. Dec 13, 2005 #1


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    I have a big physics test tomorrow:cry: , and I can't figure out how to solve this type of problem:
    2 boxes, one on a table attached to a string attached to another box hanging off the table. (the table is a frictionless plane)--It wants me to find the force of tension on the string. the diagram is as follows: (rough i know) -- the "/"and "0" represent a rod and a wheel on the end allowign the string to roll freely (again no friction)
    _____________/ |
    ....................| |
    ....................| |
    ....................| |
    ....................| [M]
    I have figured out you need to draw a free body diagram for the boxes, and figured out that Fg = Mg, but thats about as far as i got.
    Thanks in advance for the help!:smile:
    (There are no values, the answer is supposed to be in terms of M, m, and g)
  2. jcsd
  3. Dec 13, 2005 #2
    what you really need to remember is since you considering a massless pully, all it does is change the direction.

    Draw your free body diagrams for each box, nowhere should you have a FBD with a pully involved (that was what always got me). Look at the force(s) acting on your system. Try to get the big idea of what is really happening here. What is pulling on what? Try to remember that a pully just changes direction in this system.
  4. Dec 13, 2005 #3
    Force on M= Tension in string=Force on m.
    The only forces in the system are gravity on mass M and the tension of the string.. the weight of m is balanced by the normal force exerted back by the table.. Thus, since there is also no friction both on the table surface and the pully, the force on M & m shld be the same..

    tats wat i tink, correct me if i'm wrong.
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