THE MOTION TRAIN
How fast is the little train?
One layout, one journey. The whole chapter on motion is hidden along these 2 metres of track.
Scroll down and the camera will guide you ↓
01 / Mechanical motion
Is it moving?
The train leaves station A. How can you tell that it is moving?
Every 1 s, a small flag appears beside the track to mark where the front of the train is.
A change in an object's position over time is called mechanical motion.
Common mistake: a lamp switching on or water warming up does not change any position, so it is not mechanical motion.
02 / Frame of reference
Is the passenger moving or still?
At the same moment, the roadside observer and the passenger see different scenes.
Left: the roadside observer sees the tree stay still while the train passes.
Right: the passenger sees the wagon stay still while trees and houses move backwards.
With the ground as the frame of reference, the passenger moves; with the train as the frame of reference, the passenger is at rest.
Choose a frame of reference before deciding whether something moves. Motion and rest are relative.
03 / Measuring length
How far has the train travelled?
To compare speeds, first measure the distance.
The ruler's smallest division is 1 mm. At departure, the pointer on the front of the train lines up with the zero mark.
When reading, look straight at the mark and estimate one digit beyond the smallest division.
In the magnifier, the pointer stops at the third small division after 87 cm.
Record it as 87.30 cm. The last digit, 0, is the estimated digit.
04 / Uniform and changing speed
Does it keep the same speed?
A strobe camera takes a picture every 0.5 s, leaving one mark of the train front each time.
- Gaps growing: speeding up.
- Equal gaps: uniform motion in a straight line.
- Gaps shrinking: slowing down.
This trip: 2 s speeding up out of the station, 3 s at constant speed, 2 s slowing into the next. Most real motion has changing speed.
05 / Average speed
What is the average speed for the whole trip?
Two light gates split the trip into three parts. As the train passes, the display on each gate records the time.
| Segment | Distance s | time t | v = s/t |
|---|---|---|---|
| Start → gate 1 | 0.40 m | 2.00 s | 0.20 m/s |
| Gate 1 → gate 2 | 1.20 m | 3.00 s | 0.40 m/s |
| Gate 2 → end | 0.40 m | 2.00 s | 0.20 m/s |
| Whole trip | 2.00 m | 7.00 s | ? |
Average speed = total distance ÷ total time.
Model and notes
This page uses an idealised straight-line model: uniform acceleration out of the station, constant speed, then uniform deceleration. Readouts, strobe marks, gate displays and the segment table all come from the same model.
Still to come in this chapter: measuring time, errors and mistakes, speed and unit conversion, and graphs of motion.