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Homework Help: Where is the rotational axis?

  1. Sep 13, 2015 #1
    1. The problem statement, all variables and given/known data
    One side of a plant shelf
    is supported by a bracket
    mounted on a vertical wall
    by a single screw. Ignore
    the weight of the bracket.
    Find the horizontal
    component of the force
    that the screw exerts on
    the bracket when an 80.0 N
    vertical force is applied as


    2. Relevant equations

    3. The attempt at a solution
    If I choose A as rotational axis, then it should be calculated as

    net torque = 0
    (80N)(0.05m) = (x)(0.03m)

    but if a choose B as the rotational axis, then it should be calculated as

    net torque = 0
    (80N)(0.05m) = (x)(0.06m)

    So which one is correct?

  2. jcsd
  3. Sep 13, 2015 #2
    Is there a reaction @ B?
  4. Sep 13, 2015 #3
    I dont think so. What does it mean?
  5. Sep 13, 2015 #4
    Well, why don't you prove it and sum forces in each direction?
  6. Sep 13, 2015 #5
    That's what I can think of.
    (80N)(0.05m) = (n)(0.03m)
    n = 133N

    but this is incorrect and I don't know why

  7. Sep 13, 2015 #6
    You're taking moments. I want you to sum forces in each direction: ΣFx = 0 and ΣFy=0.
  8. Sep 17, 2015 #7
    Just Fx = 80 from the nail? I don't think there is a horizontal force.
  9. Sep 17, 2015 #8
    Just apply Newton's Laws and you will see that there must be a horizontal force.
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