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Moment of force

  1. Aug 17, 2011 #1
    Are the 'moment of a force' and the 'moment of a couple' are technically same or different?

    Because, the latter is called by a special term 'torque'. Both terms are confusing, as the first is measured by the product of the force and its perpendicular distance from the fulcrum where as the second is measured as the product of one the forces and the perpendicular distance from the other force.
  2. jcsd
  3. Aug 18, 2011 #2
    Hello anathu,

    Unfortunately the terms moment, couple and torque are used rather indiscriminately to describe different manifestations of 'turning effect'.

    I agree that this can be confusing, luckily the 'strenght' of the effect is the same in all cases so you can add a moment to couple to a torque (taking account of direction).

    A good clear way to think about it is

    A moment is when you have a single force acting about a point or line in a plane.

    A couple is when you have a pair of forces acting about a point or line in a plane. The forces must be equal or they have a resultant tending to cause motion.

    A torque is best reserved for three dimensional situations where you have the turning effect transferred along a shaft.

    Torque is normally used when the rotations may be more than a full circle. We would not normally talk about the moment or couple exerted by the output shaft of a motor turning at 1000 rpm.

    go well
  4. Aug 18, 2011 #3
    Thank you for this clarification.
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