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Static eq. question

  1. Jan 8, 2013 #1
    1. The problem statement, all variables and given/known data
    A diving board of length 3.00 m is supported at a point 1.00 m from the end, and a diver weighing 490 N stands at the free end . The diving board is of uniform cross section and weighs 295 N. Find the force 1) by the support 2) at the left end

    Picture Link: http://session.masteringphysics.com/problemAsset/1041644/8/YF-11-24.jpg


    2. Relevant equations

    Static equllibrium equations

    3. The attempt at a solution

    I have a ONE big doubt. I understand that the support would point upwards since it is just like normal forces. What about the force at the left end. Where will it point and why???
    Another general doubt is about equllibrium. When should we use tau net is equal to zero and when should we use the total net force is equal to zero and when should we use both .. Thanks for helping!
     
  2. jcsd
  3. Jan 8, 2013 #2

    Doc Al

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    Staff: Mentor

    What do you think? If you replaced that structure with your hand, would you have to pull up or push down?

    When in doubt GUESS. Then when you solve for the unknown force, you'll find out if you guessed the correct direction since you'll get a positive value.
    For equilibrium, both conditions must be met. The net torque equal to zero only applies with an extended body, such as a diving board.
     
  4. Jan 8, 2013 #3
    For the first part, why is there even a force at the end, there is a support. I mean the left hand could be either up or down.
     
  5. Jan 8, 2013 #4

    Doc Al

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    Staff: Mentor

    I don't understand what you mean. If you removed the structure, what do you think would happen to the board and diver?
     
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