A force can also turn
What we have learned about forces so far is not everything a force can do. A force can push an object, pull it or lift it — and a force can also turn it.
Until now, talking about a force meant talking about how it carries an object from one place to another. Yet several times a day you apply a force with no intention of taking anything anywhere. You apply it to make something turn.
In none of these does the object travel forward along a line. It turns about a fixed line — the door about the line of its hinges, the tap about its own centre. The object never leaves where it is.
The question this chapter asks
Applying a force is not by itself enough to turn something. Apply a force of the very same size to the very same door and the result changes completely depending on where you apply it. Push at the handle and the door swings open; push close to the hinges and it will hardly move at all.
Turning depends on two things, not one
The size of the force is one of them. Where you apply it is the other. This whole chapter is about the way those two are brought together.
On the next page we take a door and measure the force needed to open it at four different places. Those four readings are the foundation everything else in the chapter is built on.