What it shows
Water pressure grows with depth, so dams are thicker at the bottom. A model dam in a tray of water shows that a trapezoid profile resists the push of water better than a thin wall.
The science
Water pressure at depth h is p equals density times g times h, so it is greatest near the bottom. The push on a dam is a force that tries to slide or overturn it. A wide base and heavy mass stop sliding and overturning, which is why gravity dams are triangular or trapezoidal.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A rectangular plastic tray or tank
- Clay or soft plasticine, or wet sand
- A thin cardboard wall to compare
- A spring balance or a small weight on a thread
- A ruler
- Water and food colour
Safety
For this project: Keep the tank on a table over a tray and wipe up spills. Do not stand on wet floors. Use only a small tank so the load is safe.
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.
Shape a clay dam with a vertical thin wall across one end of the tray.
Add water slowly behind it and watch for tilting, sliding or leaks.
Mark the water depth at which the thin dam fails.
Rebuild the dam with a thick base and sloping sides, the same amount of clay.
Fill with water to the same depth and observe.
Measure the force needed to push each dam using a spring balance on a thread.
Increase the water depth in steps of 2 cm and see the effect.
Sketch the shapes and the forces of water and weight.
What you should see
The thin wall slides or tips at a shallow depth, while the sloped dam stays stable to a greater depth. Drawings show where the water pushes the hardest.
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 greater at depth.
2What is a gravity dam?
A dam that resists water by its own weight.
3What can make a dam fail?
Sliding, overturning or seepage.
4Name a big dam in India.
Bhakra Nangal, Tehri or Sardar Sarovar.
Take it further
Test a curved arch dam shape and compare with the straight dam.
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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