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 , and another force that acts downwards at 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.