The forces required to turn the steering wheel of a car. The two equal and opposite forces balance, so the revolution will not move up, down, or sideways. However, if the wheel is not in equilibrium the pair of forces will cause it to rotate. A pair of forces like that in this picture is called a couple. A couple has a turning effect but does not cause an object to accelerate.

 TO FORM A COUPLE, TWO FORCES MUST BE

1. Equal in magnitude
2. Parallel, but opposite in direction
3. Separated by distance

 We can calculate the torque of the couple in the above figure by adding the moments of each force about the Center O of the wheel:

Torque of couple = (F x OP) + (F x OQ)

= F x (OP+OQ)

= F x d

Torque of couple = one of the forces x perpendicular distance between the forces.