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Homework Help: Moments and torque question

  1. Aug 11, 2012 #1
    1. The problem statement, all variables and given/known data
    A uniform ladder AC weight 400N & length 3.5 m is held horizontally by 2 people. One person supports at A and the other at B, a distance 1m away from C.
    1. Calculate the supporting force provided by the person at A. (I managed to do this one and the answer is 120N)
    2. The person at A releases the ladder. Calculate the torque which the person at B must provide in order to keep the ladder horizontal.

    The diagram can be found here (scroll down to see diagram)

    I am having some problems with question 2. The answer to question 2 is 300Nm :)

    2. Relevant equations

    3. The attempt at a solution
    It is known that the supporting force at A is 120N when the end of the ladder (at A) has not been released, this means that the supporting force at B is 280N (400N-120N). However, I do not understand why the question asked for torque, I thought torque would require two forces which form a couple? Or am I having a gross misconception? Or does the torque here refers to the moment? If the ladder is horizontal, the ladder should be in equilibrium, hence the total upward forces should equal to the total downward forces. So does this mean that the new supporting force at B should also be 400N?

    Please include explanations along with your solution, thanks a million! :D
  2. jcsd
  3. Aug 11, 2012 #2


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    Homework Helper

    With the system as shown in the diagram, the ladder is in equilibrium. So all three forces help to provide that equilibrium.

    When person A lets go, the system is not in equilibrium anymore. The sum of the moments about any point is not zero anymore.

    For it to remain in equilibrium, the person at B will need to provide a torque to balance out the torque due to the weight.
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