Uniform and non-uniform motion

When an object moves along a straight line, its motion is called linear motion. A train on a straight track between two stations is in linear motion.

Picture the train leaving station A slowly, then speeding up, running at a steady speed from B to C, then slowing down and stopping at station D.

An object moving along a straight line at a constant speed is in uniform linear motion. So the train is in uniform motion between B and C.

If the speed keeps changing, the motion is non-uniform linear motion. The train's motion from A to B (speeding up) and from C to D (slowing down) is non-uniform.

The simple test:

• Uniform: covers equal distances in equal intervals of time.
• Non-uniform: covers unequal distances in equal intervals of time.

The book's table follows two trains from 10:00 AM to 11:00 AM, noting the distance every 10 minutes. Train X covers 20 km in every interval. Train Y covers 20, then 15, 15, 25, 20 and 25 km. So Train X is in uniform linear motion and Train Y is in non-uniform linear motion.

A note of realism: uniform linear motion is an idealisation. In real life, things seldom move at a perfectly constant speed for long — traffic, turns and stops change it. That is exactly why we use average speed.