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
1 · Read
You already know how to work out speed from distance and time. Here is the twist: the speed you measure also depends on where you are standing, or what you are moving in. That is called relative motion, and it changes how fast something looks like it is going. The place you measure from is called your frame of reference.
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 still, because both trains cover the same distance in the same time. For the same direction, subtract: 100 minus 100 is 0.
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 faster than either one moves on its own.
A car drives at 30 km/h and a bus drives at 20 km/h, heading toward each other on the same road. Their closing speed is 30 + 20 = 50 km/h, even though neither one is going that fast on its own.
A ferry sails straight ahead at 24 km/h relative to the shore. You walk forward along its deck at 3 km/h. You move at 3 km/h relative to the ferry, but 24 + 3 = 27 km/h relative to the shore.
Speeds in km/h. The ferry moves 24, you add 3 walking forward, so the shore sees 27.
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.
2 · Watch
Take it off screen
Where it sits
Learn first
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
12 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.