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Homework Help: Moments Question

  1. Jan 8, 2017 #1
    1. The problem statement, all variables and given/known data

    A uniform ladder 5m long and of mass 40kg rests with its upper end against a smooth vertical wall and with its lower end 3m from the wall on rough ground. Find the magnitude and direction of the force exerted at the bottom of the ladder. (G=9.81ms^-2)

    2. Relevant equations
    moment=force x distance from the pivot
    3. The attempt at a solution

    w(moment) = (40 x 9.81) x 2.5 =981 Nm clockwise


    therefore: reaction moment = 981Nm
    and force = 981/3 =327N anticlockwise

    Is this right or am i very far off??
  2. jcsd
  3. Jan 8, 2017 #2


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    This is not correct. It should be moment = force x moment arm. The moment arm is equal to the distance from the pivot only if the force is orthogonal to the separation from the pivot.

    What is the moment arm of the gravitational force?
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