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Torque/Static Equilibrium/Forces problem

  1. Apr 29, 2007 #1
    Torque/Static Equilibrium/Forces problem....

    I have been working on this problem for a few hours, but I just can't get it ...

    1. The problem statement, all variables and given/known data


    Problem:

    A 3.0-m-long rigid beam with a mass of 100 kg is supported at each end. An 80 kg student stands 2.0 m from support 1. How much upward force does support 1 exert on the beam?

    here is an image:

    http://photo.ringo.com/208/208651783O865465193.jpg

    ----


    2. Relevant equations

    τ=Frsinφ
    F=ma

    3. The attempt at a solution

    I figured out the torques kinda, but do i use the mass as (2/3*100)+80? or (2/3*100)+40...either way i kept getting a wrong answer....

    and the center of mass i assume is where the person is right? bec the 80kg addition would be the most massive point right?

    since the angles are 90 degrees, the sine of φ doesnt matter right?, i mean it would only affect the sign +/- of the answer....since τ=Frsinφ, and φ is either 90 or -90 degrees...

    idk...basically i just cant get this...

    i got solutions ranging from 76.666.., 1306.666... etc...
     
  2. jcsd
  3. Apr 29, 2007 #2

    radou

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    Homework Helper

    Assume the weight of the beam acting at the center of the beam, and set the sum of all torques with respect to support B equal zero.
     
  4. Apr 29, 2007 #3
    so do i pretend support 2 doesnt exist? and then just find the torque for support 1 using [80kg+(2/3*100)] as the mass?
     
    Last edited: Apr 29, 2007
  5. Apr 29, 2007 #4

    radou

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    Homework Helper

    Yes, you can pretend support 2 doesn't exist. Do you know how to draw a free body diagram? You have three forces acting - one is the weight of the bar, the second one is the weight of the person, and the third one is the force at support 1 you need to find.
     
  6. May 1, 2007 #5
    lol...yea i got it, thanks, i went to a tutor today bec i was making small errors...i for the torque of the person, i was using the weight as (person+segment of rod), when its only supposed to be person

    thanks for the help, greatly appreciated
     
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