Couple

A couple is a set of two forces that have:

• the same magnitude
• opposite directions

For example, there may be one force which acts upwards at 9N, and another force that acts downwards at 9N on another part of the object.

Points of action

Importantly, when we have a couple, the forces do not act on the same point.

Usually, they’ll act on opposite sides of a pivot, which creates a turning effect.

Transformation from a couple

• Because the forces are equal and act in opposite directions, they do not translate (move) the object.
• They do rotate the object. You can see how that works here:

    |        d
 6N |     <----->
    ⌄     
----------+----------
                ^
    <----->     | 6N
       d        |

• Both forces are the same distance from the pivot in this example.
• They have the same force, which means that the object isn’t going to go up or down.
• But the forces are in opposite directions: both are going anticlockwise around the pivot (+). That means that the plank will turn anticlockwise.