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How do you find F without the second Mass?

  1. Feb 13, 2007 #1
    I have a physics test next week for a job interview, one of the Questions I'm not sure how to answer or the formula to use:

    There is a beam with a 70kg mass at the left end.
    The fulcum point is 4m to the right, then it is 2.5m to the end of the beam.
    The question asks me to find F at the far right of the diagram.

    Not sure if my explaination will help. But how do you find F without the second Mass?

    I have not taken physics 12, so please bare with me.

    Thanks for the help in advance!

  2. jcsd
  3. Feb 13, 2007 #2


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    Staff: Mentor

    Thread moved from general Physics forum to the Intro Physics Homework Help forum. jdub88, Welcome to the PF, but please keep in mind that homework and coursework problems (and review problems like this one you are doing for yourself) must be posted in the Homework Help forums, and you must show some of your own work in order for us to help.

    Now having said that.... Have you learned how to draw free body diagrams (FBDs) yet? If so, you just draw the lever, fulcrum and mass on the left end, and sum up all the forces acting on the beam to equal zero (because nothing is moving when it is all balanced). You ask about a balancing mass at the far right of the beam, but it doesn't have to be a mass that is exerting the downward force on the right end, does it? It could be anything. You are just asked to find the force F down on the right end that would cause the beam to be balanced.

    Have you studied torques? If an object is not rotating, the sum of the torques (about the fulcrum in this case) has to equal zero....
  4. Feb 13, 2007 #3
    A force acting farther away from the fulcrum will have more "leverage" or torque. Basically, force1*distance1 = force2*distance2 because the lever is not moving
  5. Feb 13, 2007 #4
    I did draw a FBD, which helps but I can't figure out a formula to use..I kept thinking F=m*a. So without m, how could I find the force acting on the lever. So the distances how nothing to do with the question?
    Therfore F= 0.
    I have not studied torques, but I will look up the concept.

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