Chapter 3

Newton’s Laws

In the previous chapter we studied the vocabulary and equations that describe
motion. Now we will learn why things move the way they do; this is the subject of
dynamics.

An interaction between two bodies, a push or a pull, is called a force. You see
examples of forces every day. If you lift a book, you exert an upward force (created
by your muscles) on it. If you pull on a rope that’s attached to a crate, you create a
tension in the rope that pulls the crate. When a skydiver is falling through the air,
the earth is exerting a downward pull called gravitational force, and the air exerts
an upward force called air resistance. When you stand on the floor, the floor
provides an upward, supporting force called the normal force. If you slide a book
across a table, the table exerts a frictional force against the book, so the book
slows down and then stops. Static cling provides a directly observable example of
the electrostatic force.