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| "Torque" and "moment" are synonymous. A couple is a special
| type of torque.
|
| About any origin, there is a torque associated with each
| force. If the line of action of a force passes through the
| origin, then the torque associated with that force is zero.
|
| A couple is the net torque associated with two forces that
| are equal and opposite.
Its important to note that both external forces that constitute the
couple must be acting on the *same* object. This is why 3rd law
interaction pairs do not form a couple. They may have the same line of
action; but this makes for a triviality.
The significance is that the torque computed for a couple is independent
of the point about which you calculate the torque. Which is physically
inconsequential, but can be of calculational importance; allowing you to
choose a point about which to calculate torques that minimizes the
difficulty of the calculation.