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Homework Help: Rod problem

  1. Apr 14, 2015 #1
    1. The problem statement, all variables and given/known data

    Two like parallel forces 20N and 30 N act at the ends A and B of a rod 1.5m long. The resultant of the forces will act at a point
    1 90 cm from A
    2 75 cm from B
    3 20 cm from B
    4 85 cm from A
    2. Relevant equations
    Torque=Force * Perpendicular distance

    3. The attempt at a solution
    In the solution for the problem, I see that the torque due to the forces are equated
    and then x has been calculated.
    I dont understand this approach. What is the point in equating torques? It has not been mentioned that the net torque on the rod is zero. Then how can we assume that the torques due to the forces are equal?
  2. jcsd
  3. Apr 14, 2015 #2


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    Science Advisor
    Homework Helper
    Gold Member

    the magnitude and direction of the resultant force is given by the vector sum of the forces. The location of the resultant force must be such that the moment of the resultant force about any point is the same of the sum of the moments of the individual forces about that same point. If you choose the location of the resultant force as the point about which to sum moments, then since the moment ofthe resultant force about that point is 0, then so must be 0 the sum of the individual force moments. The fact that when doing it this way gives 0 moment does not imply the net moment about any point is 0.
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