A force needs two objects

Push a table. Your hand is one object pushing on another object, the table. The two are interacting.

Look back at every action in Table 5.1 and count the objects. There are always at least two. Your hand on its own, or your cheek on its own, makes no force. A force appears only when two objects interact.

So the full definition is: a force is a push or pull on an object that results from its interaction with another object.

The SI unit of force is the newton, written with a small n. Its symbol is N.

Now the second half of the idea. When you pushed the table, did you feel something pushing on your hand? The moment you stopped, that feeling vanished. Whenever two objects interact, each one feels a force from the other, and when the interaction stops, neither does.

One question to keep in mind: if something is lying still, does that mean no force is acting on it? Not necessarily. It may mean the forces on it are balancing each other out. You will learn about balanced forces in higher classes.