Two forces instead of one
To turn a body with a single force, that body has to be held fixed somewhere.
Every case we have looked at so far had such a holding. The door was held by its hinges; the wooden strip was held by a screw. Without it the force would simply have pushed the object along instead of turning it.
A body with no pivot
Think of a book lying free on a table. Push one corner of it and it does turn a little — but it slides forward at the same time. There is no way to make it turn while staying where it is.
What if there were two forces?
Two forces acting in opposite directions, though, can turn a body whether it is held in place or not held at all.
You use this constantly without thinking about it. A steering wheel is turned with two hands pulling opposite ways, not with one. A tap is turned by two fingers pushing against each other.
The idea of this section
Two equal forces acting in opposite directions can turn a body — while pushing it nowhere at all. A pair of forces like that has a name of its own.
On the next page we show it with a wooden strip and two Newton balances.