1. The relative motion of objects.

2. 2 m/s 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

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

4. This is known as relative motion
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 1m/s relative to the walking person.

5. 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

6. Now we have two trains moving past each other
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 A
Train B
2 m/s

7. 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.

8. What is their relative speed?
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?

9. 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?

10. (Hint: There are 1600 metres in every mile)
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)

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

12. Extension! You are sitting still, your motion is 0 m/s,
But the Earth is rotating at about 900mph
And the Earth is moving through space at about mph
How fast are you moving?
Extension!