What it shows
The wheel pushes water backwards and the water pushes the boat forward, as in Newton’s third law. The twisted rubber band supplies the energy.
The science
When the paddles turn, they push water backward. By Newton’s third law, the water pushes the paddles, and so the boat, forward with an equal and opposite force. The twisted rubber band stores energy that turns the wheel. The bottle floats because its shape holds a lot of air, so it displaces enough water to support its weight.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 500 ml plastic bottle for the hull
- Two ice-cream sticks for the paddle frame
- Four bottle caps or card pieces as paddles
- A thick rubber band
- A skewer for the axle
- Tape and a tub of water
Safety
For this project: An adult must cut the bottle. Use the boat only in a tub or a shallow pool with an adult nearby.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Tape two sticks along the sides of the bottle so they stand out past the back.
Fix the skewer across the ends of the sticks as an axle.
Attach four sticks or caps around the skewer like a wheel.
Loop a rubber band from the front of the bottle to the middle of the axle.
Wind the paddle backward to twist the band.
Place the boat in the tub and release the wheel.
Measure the distance travelled and time.
Change the number of paddles and compare.
What you should see
The boat paddles forward across the tub, and more winding gives a longer run.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the boat move forward?
The paddles push water back and the water pushes the boat forward.
2Which law is this?
Newton’s third law.
3Why does the boat float?
It displaces water equal to its weight.
4Where does the energy come from?
The twisted rubber band.
Take it further
Try two bottles as a catamaran and see if it travels straighter.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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