The relative motion of objects Imagine you are

The relative motion of objects.

Imagine you are standing still on a railway platform. A train slowly moves past you and covers 10 metres in 5 seconds. How fast is it moving? 2 m/s

Now imagine that you start to walk in the same direction as the train. 1 m/s You walk at 1 m/s. 2 m/s How much faster than you is the train travelling? 1 m/s

This is known as relative motion. It is a measure of how fast something is travelling compared to something else. For example: • The train is moving at 2 m/s relative to the platform. • It is also moving at 1 m/s relative to the walking person.

This can sound a bit confusing… If you are seated in a train travelling at 25 m/s, then; • Your relative motion to the railway line is 25 m/s • But your relative motion to your seat is 0 m/s

Now we have two trains moving past each other. For a passenger on train A, how fast does train B appear to be approaching? 5 m/s 3 m/s Train B 2 m/s Train A

Key Idea • For two objects moving in the same direction, the relative motion is simply the difference between their speeds. • For two objects moving towards each other, the relative motion (sometimes called the approach speed) is simply the two speeds added together.

Challenge 1 Two cars are approaching each other. One is travelling at 30 mph whilst the other is travelling at 50 mph. What is their relative speed?

Challenge 2 Two cars are travelling in the same direction on a motorway. Car A is travelling at 20 m/s and car B is travelling at 30 m/s. It takes 30 seconds for car B to overtake car A, How much further does car B travel during this time?

Challenge 3 Two cars are approaching each other. One is travelling at 20 m/s whilst the other is travelling at 30 m/s If they are 5 miles apart how long will it take until they pass each other? (Hint: There are 1600 metres in every mile)

Answers: • Challenge 1: 80 mph • Challenge 2: 300 metres • Challenge 3: 160 seconds

Ex te s n ! n • You are sitting still, your motion is 0 o i m/s, • But the Earth is rotating at about 900 mph • And the Earth is moving through space at about 69 000 mph How fast are you moving?