A train starts: why the wagons jerk one by one
A locomotive pulls five wagons with slack couplings. They start not together but one after another — a wave of jerks.
2D
Age 7+10 min
Momentum & collisionsMachines & mechanisms
Loading simulation…
Press “Run”. Bodies move by the laws of physics: gravity, friction, elasticity.
What the experiment shows
There is slack between the wagons. Until the coupling is taut the wagon stands still. Then it suddenly gets speed, and the slack is taken up at the next one.
How to run it
- Press “Run” and watch the wagons start.
- Make the couplings shorter — the jerks get smaller.
- Make the last wagon heavy.
Questions to discuss
- Why do passengers at the rear feel the jerk more?
- Why does a driver first back up before pulling a heavy train?
The science
Force passes along the couplings only when they are taut. Each jerk is a short blow that passes momentum. The locomotive gets wagons moving one at a time, which makes a long train easier to start.
Try it at home
Tie several toy cars with slack threads and pull the first one — you’ll see the same wave.
What to change in the editor
- Change the locomotive speed.
- Add another wagon.