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Homework Help: Torque Question

  1. Jul 14, 2013 #1
    1. The problem statement, all variables and given/known data

    A ladder 6 m long and weighs 120 N. It leans against a smooth wall of negligible friction making an angle of 50° with the horizontal. The coefficient of friction between the floor is 0.5. How far up the ladder can an 800 N worker climb before it starts to slip?

    I ended up with the equation:

    [itex]d*800\sin 40 = 0.5*2000*3*sin 50[/itex]
  2. jcsd
  3. Jul 14, 2013 #2


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    why do you have sin(40) and sin(50). would one of those not be cos?
  4. Jul 14, 2013 #3
    But for torque, isn't it:
    distance * force * sin x
  5. Jul 14, 2013 #4


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    well. if you draw the FBD the 40 degree angle is in one direction and the 50 is in the other direction.
  6. Jul 14, 2013 #5
    He oneplusone! Always start these typs of mechanics question by drawing FBD. If you already have drawn it, show it to us!
  7. Jul 14, 2013 #6
    Sorry, I don't really understand what you're saying. Can you write down the first set of equations so I can see?

    EDIT: I drew a FBD, but am unable to post it here.
  8. Jul 14, 2013 #7
    Start like this. Let the man be climbing a distance d on the ladder from the ground.

    How many forces do see on the ladder?
    What is the restoring Force?
    At the point of slipping these forces will balance.
  9. Jul 14, 2013 #8
    I chose the CM of the ladder as the pivot point.

    I saw the frictional force (u*Normal force) and the weight of the worker
  10. Jul 15, 2013 #9
    There are three Normal Forces on the ladder, one gravity and one Friction Force. In total there are Five Forces. Now balance the net Forces(in x and y direction separately) and net Torque. You will get three equations for three variables. Solve it.
  11. Jul 16, 2013 #10


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    A 'normal' force means a force at right angles to a contacting surface. There are two contacting surfaces. Maybe you mean two normal forces, two gravitational forces, and a frictional force?
    oneplusone, please post your working, not just the equation you ended up with.
  12. Jul 16, 2013 #11
    No i meant three Normal Forces. The person standing on the ladder will be perpendicular to the ladder surface(on one of the cyllindrical sticks). Two normal and two gravitational Force will give same answer but the Normal Forces on the ladder are three.
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