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Lines of action of forces and net torque

  1. Mar 26, 2009 #1
    I have a question.

    For example there are three forces of diff magnitudes and are in three diff directions (not oppo directions) acting on a body. if the lines of action of the 3 forces pass through a common point, why the net torque abt that pt is zero?

    Can someone help me pls?
    Thanks!
     
  2. jcsd
  3. Mar 27, 2009 #2
    k im not sure what u asking but , u could add up all the x and y components
    of the vectors and get the resultant vector.
     
  4. Mar 27, 2009 #3
    Erm. What i want to ask is just this:

    Why when the lines of action of 3 forces pass through a common point, the net torque abt that common point is zero?
     
  5. Mar 27, 2009 #4
    I think I understand your question. Suppose we have we have a rod x units long, centered at x = 0. Suppose there is a force +F at +x/2 and a force -F at -x/2. There is no net force on the rod, but there is a torque about the point x=0. However, if the two opposing forces were both applied at x=0, then there is no torque about the point x=0. Even if the two opposing forces were applied at x=0 but not equal, there still is no torque about the point x=0. Is this your question?.
     
  6. Mar 27, 2009 #5

    tiny-tim

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    Hi makeAwish! :smile:
    It even works for just one force …

    the net torque must be zero about any point if the force goes through that point …

    torque (or moment) = force x distance, and distance = 0. :smile:

    (btw, this is why we always take moments about hinges etc, because we want to simplify the problem by leaving out the force at that point :wink:)
     
  7. Mar 27, 2009 #6
    oh ya... each force produces zero torque cos r is 0, so the sum of it at that point is still zero..

    haha. thanks a lot!! :)
     
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