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Homework Help: Tension force, acting about a pivot

  1. Feb 6, 2010 #1
    1. The problem statement, all variables and given/known data
    A mass of 20kg is hanging on a rope. Another cord is tied to the rope part way down and is attached to a wall. This causes the rope to make a 60 degree angle to the ceiling.
    a) Calculate the tension force in the upper section of the rope.
    b) Calculate the force exerted by the cord on the wall.

    2. Relevant equations
    Tension=SinX * F

    3. The attempt at a solution
    I believe part a) is (where gravity = 9.8):
    Tension = Sin60 * (20 * 9.8)
    Tension = 170N (2sf)

    I'm not sure what to do for part b), hence why I've posted this.
  2. jcsd
  3. Feb 6, 2010 #2
    To make the rope non-vertical, you should ADD force to the system (through the cord).
    It means the force cannot be less than m*g.
    It would be helpful to sketch up a drawing, partly because as far as I can see not all variables are given, most specifically the angle of the cord (it is maybe just because I am not a native English speaker). Let's take it to be horizontal for now.

    So you have three forces canceling each other:
    - the force on the vertical part of the rope pulled by the weight
    - the force on the 60 degree part of the rope
    - the force on the horizontal cord.

    One of the forces is known. The other two should cancel each other's horizontal component.
    So you can determine the 60 degree rope's horizontal component based on its vertical one and its angle.
    The non-horizontal cord case is a bit more difficult, but you can still write equations for the horizontal and the vertical component, and solve it if the angle of the cord is known.
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