# What is the difference among moment, torque and couple?

1. Oct 18, 2012

### qpzm77gg

What is the difference among moment, torque and couple?

Moment = Fd
Torque = Fd
Couple mean two forces have same magnitude but opposite direction.

What is the relationship among them?

2. Oct 18, 2012

### Philip Wood

Moment and torque mean exactly the same. You must always specify the point about which you are finding the moment (or torque) of a force. A given force will have different moments (or torques) depending on your choice of point.

A couple is a pair of equal and opposite forces not acting in the same straight line. What's special about a couple is that the total torque of such forces is the same about whichever point we choose. It is given by
$$G=Fd_{perp}$$ in which $d_{perp}$ is the perpendicular separation of the lines of action of the two forces. This is easy to show.