Concept preview

What you will be able to do

1. 01Distinguish mass from weight
2. 02Calculate weight with F_g = mg
3. 03Apply Newton's law of universal gravitation and the inverse-square rule

This chapter is all about forces, so let's define one cleanly. A force is a push or a pull on an object. It's a vector — it has a size (measured in newtons, N) and a direction — and it can do two things: change an object's motion (speed it up, slow it down, turn it) or change its shape. Some forces act only on contact (a push, friction, a spring); others reach across empty space (gravity, magnetism). Gravity is the field force we start with, because it acts on absolutely everything.