Couples around us
A couple is not just an idea in a book. You apply one several times a day.
A water tap
Figure to be added
A couple acts on the head of a tap as it is opened and closed. Your fingers push on opposite sides of the head in opposite directions, and together the two turn it.
The head is not dragged sideways — it only turns. That is what a zero resultant force gives you.
A screwdriver
Figure to be added
What we apply to the handle when driving a screw in or out is a couple. The fingers on one side of the handle push one way and those on the other side push the opposite way.
Why are the handles fat?
The thicker the handle, the greater the distance d between the two lines. Since M = F × d, the same grip delivers a larger moment. That is why a stiff screw calls for a screwdriver with a fat handle.
A steering wheel
Figure to be added
In turning a steering wheel, applying two equal forces in opposite directions from either side is easier than turning it with a single force from one hand.
What all three share
Tap, screwdriver and steering wheel — in every one of them you take hold not at the centre but at two points as far apart as they will go. That is the gain in moment that comes from making d large.
Can you find more?
Write down two more everyday situations in which a couple acts. For each one, say at which two points the forces are applied.
A hint: think of anything you turn by gripping it with two hands or two fingers.