Relative Motion
Science: KS3, topic 3.2.11
Relative Motion
Objects seem to move at different speeds depending on whether or not you yourself are moving. Have you ever looked out the window of a train, thinking you were leaving the platform, but it was another train moving instead?
Relative motion
- Relative motion is the speed of something compared to the observer.
- If you are in a car and go past people at a bus stop, you may not see them very well because you are moving and they are still.
- If you overtake a motorbike in your car, and look at the rider, it may seem you spend longer going past them than the people at the bus stop.
Calculating relative motion
- If two objects are moving in the same direction, the relative motion is:
- Fastest speed - slowest speed.
- If objects are moving towards each other (in opposite directions), the relative speed is:
- Object 1's speed + object 2's speed.
- This why a head-on collision is worse.
Relative motion - train calculation
- A train that travels at 50m/s from the East is heading towards a train that is traveling at 70m/s from the west. What is the relative motion?
- 50+70 = 120m/s.
Test yourself on Relative Motion
Why is a car crash worse if both cars are travelling towards each other?
A train travelling at 50 m/s from the east is heading towards a train travelling at 70 m/s from the west. What is their relative motion?
Car A and car B travel in the same direction. Car B is faster than car A. If car B speeds up even more, what happens to the relative motion between the two cars?