Relative motion: it depends who's watching

Science · Forces & Motion · ages 11-12

Name ______________________ Date ____________

You are on a train moving at 50 km/h. You walk toward the front at 5 km/h. What is your speed relative to the train?
- 5 km/h
- 55 km/h
- 50 km/h
- 45 km/h
What is a 'frame of reference' in physics?
- Where you measure motion from
- The top speed an object can reach
- The total distance covered by a trip
The speed you see for a moving object never depends on your own motion.
Circle one: True False
Two trains travel in the same direction at the same speed. What does a passenger on one train see when looking at the other train?
- It looks like it isn't moving
- It looks like it's speeding up
- It looks like it's going backwards
Two cars drive in the same direction. One goes 60 km/h, the other 40 km/h. What is their relative speed?
- 20 km/h
- 100 km/h
- 60 km/h
- 40 km/h
Two cars drive toward each other. One goes 60 km/h, the other 40 km/h. What is their relative speed?
- 20 km/h
- 60 km/h
- 100 km/h
- 40 km/h
Your train and another train travel side by side at the same speed, in the same direction. Your train stops at a platform while the other keeps its speed. Why does the other train look faster?
- You've stopped moving, so now you see all of its speed
- The other train sped up the moment you stopped
- Trains always speed up near platforms
Car A drives toward Car B at 50 km/h. Car B drives toward Car A at 30 km/h, on the same road. What is their closing speed?
- 20 km/h
- 80 km/h
- 1500 km/h
A cyclist and a car head straight toward each other on the same road. The cyclist rides at 15 km/h and the car drives at 45 km/h. What is their closing speed, in km/h?
Answer: ______________
Zara says: 'Two cyclists both ride at 20 km/h in the same direction, so their relative speed is 40 km/h.' What mistake did she make?
- She used the wrong speed unit
- She mixed up distance and time
- She added the speeds instead of subtracting them for same-direction motion

Answer key

For grown-ups. Fold this page away before handing over the rest.

Relative motion: it depends who's watching W1-mt_-OndzpVsrv-s1

5 km/h · Relative to the train, you only consider your walking speed on the train. The train's motion does not affect your speed relative to the train. You walk at 5 km/h relative to the train.
Where you measure motion from · A frame of reference is the position or motion you use to judge speed and direction from.
False · That is false. Your speed and direction can change the speed you measure for another object.
It looks like it isn't moving · Since both trains cover the same distance in the same time, the gap between them never changes, so it looks still.
20 km/h · When objects move in the same direction, subtract the slower speed from the faster speed. 60 km/h - 40 km/h = 20 km/h.
100 km/h · When objects move toward each other, their relative speed is the sum of their speeds. 60 km/h + 40 km/h = 100 km/h.
You've stopped moving, so now you see all of its speed · The trains first had zero relative speed. When your train stops, you measure the other train's unchanged speed relative to the platform.
80 km/h · When two things move toward each other, add their speeds: 50 + 30 = 80 km/h.
60 · Moving toward each other, the speeds add together: 15 + 45 = 60 km/h.
She added the speeds instead of subtracting them for same-direction motion · For objects moving the same direction, you subtract the speeds, not add them: 20 - 20 = 0 km/h relative speed.