A bus and a car, catching up

What the experiment shows

The bus left a few hours earlier. The green zone stands where the car will catch it, and the S caption shows the shrinking gap.

How to run it

1. Run the scene: the chaser gets closer and the S caption shows the gap.
2. Enter your answer in the Task panel and press “Check”.
3. Change the speeds or the head start with the sliders: will the fast one catch up sooner or later?

Questions to discuss

• Why does the gap shrink by the difference of the speeds, not the sum?
• What happens if the speeds are almost equal?

The science

While one moves ahead the other chases: every hour the head start shrinks by the difference of the speeds, the closing speed in a chase. The chase time is t = head start / (v₂ − v₁).

Try it at home

Let a friend start 10 steps ahead while you walk a bit faster: how many steps do you need to catch up?

What to change in the editor

• Open the example in the editor and add a third participant who leaves even later.
• Swap the speeds so that catching up is impossible and explain why.