Relative Motion

Activities to try

1. 1

   Switch the observer on the same train scene

   Why it works: The same motion looks different from different frames; nothing about the trains changes when you switch observers — only who is watching. Motion is always 'relative to' a chosen frame, not an intrinsic property, and the Earth/ground is just one frame among many.

   1. Watch a scene of two trains on parallel tracks beside a platform, with all objects at rest before the motion begins.
   2. Choose a viewpoint — RIDE TRAIN A, RIDE TRAIN B, or STAND ON PLATFORM — and observe how the SAME physical motion is re-described from that viewpoint: e.g. riding Train A, the platform and scenery slide backwards and Train B (moving at the same speed and direction) appears frozen alongside; on the platform, both trains move.
   3. For each viewpoint, read the apparent-velocity labels and relative-speed readout, which use explicit subscripts (e.g. 'Train B relative to Train A', 'train relative to platform').
   4. Switch to an 'approaching/oncoming' setup and compare: two trains going the same way show a small relative speed; two oncoming trains show a large one.
   5. Try the same frame-switch reasoning on other everyday scenarios — a car passing you on the road, or a boat crossing a river (its motion looks different relative to the water than relative to the bank) — to see the idea is not special to trains.
2. 2

   Walk-across-the-room: viewpoint vs background

   Why it works: A reference frame is set by the observer's own motion, not by the background or scenery. Observers moving differently (the seated class and the pupil walking alongside) give different but equally valid descriptions of the same walk; observers at rest relative to each other share a frame, even if facing opposite ways makes their left/right words differ.

   1. One pupil stands still to act as 'the background'; the teacher (or another pupil) walks steadily between that pupil and the class, while a second pupil walks alongside the walker at the same pace as a moving observer.
   2. The seated class describes the walker as moving across the room; the moving observer describes the walker as staying beside them. The background pupil may say 'to the left' where the class says 'to the right' only because they face the other way: they are at rest relative to the class, so they share the class's frame and agree the walker is moving at the same speed.
   3. Compare the two descriptions of the exact same walk and discuss why they differ.
   4. State explicitly that the reference frame is set by the observer's own motion, not by the background pupil, the scenery, or which way the observer is facing.
   5. Extend: walk two pupils side by side at the same steady pace and have the class predict how fast each walker sees the OTHER moving, and why, before asking the walkers.
3. 3

   Compute relative speed: add or subtract?

   Why it works: Relative speed is found by subtracting the speeds when objects move the same way and adding them when they move towards or away from each other in opposite directions; the same-speed same-direction case gives zero relative speed (appears stationary).

   1. Work through the same-direction case: two trains (or cars) travelling the same way at the same speed — compute the relative speed and confirm it is zero, so each appears stationary to the other.
   2. Work through two cars driving side by side at the same 60 mph: state how fast each appears to move as seen from the other, and explain why it looks stationary.
   3. Work through an oncoming case: two cars approach each other, one at 50 and one at 40 — compute how fast they close on each other (50 + 40 = 90), then redo it for both going the same way (50 − 40 = 10, closing if the faster is behind or separating if it is ahead).
   4. Optionally use a number line: mark where each car is after one hour from where they started. In opposite directions the distance between them changes by both distances together (50 + 40 = 90), while in the same direction it changes only by the difference (50 − 40 = 10).
   5. State the rule in your own words: same direction → subtract the speeds; towards or away in opposite directions → add the speeds.