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Homework Help: Need some help with forces on a dangling prism Internal Assessment

  1. Mar 12, 2017 #1
    1. The problem statement, all variables and given/known data
    I made an apparatus of a rectangular prism dangling from a vertical string from one side while the other is touching the ground. The relationship I'am trying to find is between the angle of the prism relative to the ground and the normal force exerted up wards. I am stuck on how I should find the relationship between the normal force and the angle of the object relative to the ground, What I have found out so far is that Ft + Fn = Fg because the object is not moving and string is vertical so net force is zero. I think it's similar to a static ladder against the wall problem but I don't know if the horizontal normal force of a ladder touching the side of a wall functions the same as the object hanging from a string while still touching the ground. I just need some input on how Ft and Fn differ when on side is hanged from a vertical string vs when one side is leaning against the wall.

    2. Relevant equations

    3. The attempt at a solution
    I attempted to use torque by stating net torque was zero but have no idea to proceed when there is normal force on one side and tension force on another.
    Last edited by a moderator: Mar 12, 2017
  2. jcsd
  3. Mar 12, 2017 #2


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    Welcome to PF!
    Yes, good.

    What if you pick the origin for calculating torques at the midpoint of the edge that's in contact with the ground? Think about the lengths of the lever arms for the three forces with this choice of origin.
  4. Mar 12, 2017 #3


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    I do not understand why that should create any difficulty. Please post your attempt and say why you are stuck.
  5. Mar 12, 2017 #4
    Cos theta cancels out, the relationship I am trying to find is how does the change in theta affect the normal force of the object but since theta cancels I am not sure how to continue with this problem.
  6. Mar 12, 2017 #5
    The horizontal distance from point where the rope is attached, to the center of mass is not equal to the horizontal distance from the center of mass to the point where the prism (isn't this a block?) touches the floor. The tickness is important. If you used a flat plate your computation would be correct. Fn would be Fg/2 and would not depend on phi,
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