Couple

A couple is a set of two forces that have:

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.

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.

Question	Answer
What is a couple?	A set of two forces that have the same magnitude and opposite directions
Give an example of a couple	One force acting upwards at 9N and another acting downwards at 9N on a different part of the object
Where do the forces in a couple act?	They do not act on the same point; usually on opposite sides of a pivot
What is the translational effect of a couple?	They do not translate (move) the object
What is the rotational effect of a couple?	They do rotate the object
In the example diagram, both forces are the same distance from the pivot and have the same force — what does this mean for the object’s vertical motion?	The object is not going to go up or down
In the example diagram, both forces act anticlockwise around the pivot — what happens to the plank?	The plank will turn anticlockwise