What it shows
Water pressure grows with depth, so the base of a dam must be thicker than the top. The model has a spillway and a sluice gate and shows how stored water gives energy.
The science
The pressure at a depth h in water is the density times g times h, so it grows steadily with depth. The force on the wall of a dam is therefore larger near the bottom, and the dam must be broader there to stay put. A dam also stores water at a height, giving it gravitational potential energy that can be turned into electricity as it falls through turbines.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A large plastic tub or tray
- Corrugated cardboard covered with plastic film
- Clay or plasticine to seal gaps
- A bottle with water
- Small paper boats
- Waterproof tape and a ruler
Safety
For this project: Work over a tray. An adult must cut the cardboard and clean up the spilled water.
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.
Make two cardboard walls, one of uniform thickness and one sloping thicker at the base.
Cover both with waterproof tape to keep them from going soft.
Place each wall across a tub so it holds back water.
Seal the sides and bottom edges with clay.
Pour water slowly behind each wall to the same level.
Push each wall gently from behind with a hand and compare how much it moves.
Cut a small notch near the top as a spillway and a hole at the base with a plug as a gate.
Open the plug and notice the stronger jet from the lower hole.
What you should see
The base-thick wall resists the water better, and the lower hole sprays farther than the upper one.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is a dam thicker at the bottom?
Pressure is greatest at depth.
2What does a spillway do?
It lets extra water flow out safely.
3What energy does stored water have?
Gravitational potential energy.
4Name a big dam in India.
Bhakra Nangal or Tehri.
Take it further
Compare the jet distance at three different depths and plot it.
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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Physics Working Models Explained
Twenty classic models, how each works and how to make it
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Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present