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Homework Help: Beam, wall, momentum

  1. Nov 7, 2014 #1
    1. The problem statement, all variables and given/known data
    Homogenous beam rests on smooth wall such that it forms an angle of 60 degrees with the ground (picture is rotated). A body of mass m = 60 kg rests 7 m from the end of beam which lies on ground. Mass of beam equals 15kg and length equals 10 m.
    What is the minimal friction factor such that beam remains in same position?

    2. Relevant equations
    Torque = Force x distance

    3. The attempt at a solution
    I guess that total moment needs to be 0 but I can't go further then that. Also I don't know what the center should be. I know that the forces are gravitational of body, beam, and friction of beam and ground.

    Attached Files:

    Last edited: Nov 7, 2014
  2. jcsd
  3. Nov 7, 2014 #2


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    You have messed up your terminology and equations..

    What are the units for momentum? Force? Distance? What is the equation for work? What is the equation for torque?

    Do objects with zero velocity have momentum?
    If you want to find static equilibrium (ie case when it does not move) is momentum relevant?
  4. Nov 7, 2014 #3

    Simon Bridge

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    Homework Helper

    ... this is not correct.
    Perhaps you are thinking of the "moment"? Momentum is velocity times mass.

    There are two conditions for a system to be static. One is that the sum of the moments must be zero. What is the other one?
    Have you identified the forces on the beam? Have you drawn a sketch with the forces, labelled, drawn on it?
  5. Nov 7, 2014 #4
    Ahh...I have problems translating it in English, I understand it. Anyways I thought of force x distance whatever it is called. I will edit it now.
  6. Nov 7, 2014 #5
    Ok, I know that net torque of all forces and net force must be zero.
    I draw the following forces:
    1)gravitational force of beam (located in the middle of beam)
    2)gravitational force of body ( exactly where the body is)
    3) friction (acts horizontally?)

    Then, I tried to write these forces as components of which some are parallel to beam and those which are forming right angle with beam.

    But I think that problem for me is that in past problems I've done I have always had some point that was static and it was easy to calculate torque. Now I don't see that point or what?
  7. Nov 7, 2014 #6
  8. Nov 8, 2014 #7
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