The boat's own speed

What the experiment shows

Two boats run side by side: one downstream, one upstream. Each shows its speed, and the task checks the own speed.

How to run it

1. Run the scene: the boats go with and against the current, and each shows its speed.
2. Enter your answer in the Task panel and press “Check”.
3. Set the current to 0 and compare the speeds: what changed?

Questions to discuss

• Why is the current added downstream and subtracted upstream?
• What happens if the current is faster than the boat's own speed?

The science

Downstream the boat's speed is its own speed plus the current, upstream their difference: v down = v₀ + vc, v up = v₀ − vc. The downstream speed exceeds the upstream speed by 2vc.

Try it at home

Float a paper boat in a stream or in a tub with a hose current and time it over one metre.

What to change in the editor

• In the editor set the current so that the boat upstream stands still.
• Add a second boat with a different speed and make a race.