Concept preview

What you will be able to do

1. 01Distinguish distance from displacement and speed from velocity
2. 02Calculate average speed and average velocity
3. 03Read position–time graphs and interpret their slope

Before physics can explain why things move (that's forces, coming in Chapter 3), it has to describe how they move — precisely. This branch of physics is called kinematics, the mathematics of motion. In everyday life we get away with fuzzy words: 'it's not far', 'she's going fast'. Physics can't. We need numbers, units, and — crucially — directions. This whole chapter is built on four quantities: distance, displacement, speed, and velocity. Get these four rock-solid now and every motion problem for the rest of the year becomes easy.