A boat between two piers
A boat goes between two piers in one time downstream and another upstream. Find its own speed.
2D
Age 10+10 min
Motion problems
Loading simulation…
Press “Run”. Bodies move by the laws of physics: gravity, friction, elasticity.
What the experiment shows
Both boats cover the same distance between the piers: the upper downstream, the lower upstream. The distance is not given, you have to find it.
How to run it
- Run the scene: the boats go with and against the current, and each shows its speed.
- Enter your answer in the Task panel and press “Check”.
- 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.