Relative Motion
Velocity depends on the observer's frame of reference.
About this topic
Velocity depends on the observer. Someone walking at 1 m/s along a train moving at 20 m/s has a velocity of 21 m/s relative to the ground but 1 m/s relative to the train. To find relative velocity, add or subtract vectors depending on direction.
The general rule is velocity of A relative to C equals velocity of A relative to B plus velocity of B relative to C. In the same direction, speeds add; in opposite directions, they subtract. For motion at angles, use vector addition with components or the Pythagorean theorem.
A swimmer crossing a river, or a plane in crosswind, ends up with a path different from where they point. Example: a boat heading straight across at 4 m/s in a river flowing at 3 m/s travels at 5 m/s relative to the bank, drifting downstream at an angle of about 37 degrees. Pilots must crab into the wind to hold their course.
Tip: always label which object is measured relative to which, and draw a vector diagram. A common mistake is to treat a speed given relative to water or air as if it were relative to the ground.
Key ideas
- Add or subtract velocities
- A boat crossing a current drifts downstream
- All motion is relative
Quick check
Use cases: where Relative Motion helps
- Homework: review the key idea behind Relative Motion before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Relative Motion without notes.
- Visual learning: play with the interactive lab to see how Relative Motion changes when you adjust its inputs.
- Teaching a friend: explain Relative Motion in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Relative Motion?
Velocity depends on the observer's frame of reference.
What will I learn in Relative Motion?
Add or subtract velocities. A boat crossing a current drifts downstream. All motion is relative.
Is Relative Motion part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Relative Motion lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Relative Motion?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.