Relative Motion
Explain relative motion — how the apparent speed and direction of an object depends on the observer's own motion — using everyday examples such as trains and cars passing
What a learner can do afterwards
- Explains why a train moving at the same speed in the same direction as another appears stationary to passengers on that train
- Calculates relative speed when two objects move towards or away from each other
- Explains why the frame of reference matters when describing motion
The lesson
You already know how to work out speed from distance and time. Here's the twist: the speed you measure also depends on where you're standing, or what you're moving in. That's called relative motion, and it changes how fast something looks like it's going.
Two trains run on parallel tracks, both moving at 100 km/h in the same direction. Someone standing on the platform sees both trains speeding past. But a passenger looking out the window at the other train sees it looking almost still, because both trains cover the same distance in the same time.
To compare two moving things, check their directions first. Same direction: subtract the slower speed from the faster one. Heading toward each other: add the speeds, because the gap between them closes twice as fast.
A car drives at 60 km/h and a bus drives at 40 km/h, heading toward each other on the same road. Their closing speed is 60 + 40 = 100 km/h, even though neither one is going that fast on its own.
What speed you observe always depends on your own motion: add speeds for things heading toward each other, subtract them for things heading the same way.
Watch it
Where it sits
Learn first
This opens up
Nothing builds on it yet.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.