Week 16 · Theme 3 Protecting Our Planet, Unit 3 Energy and Fuels: Get Started, Concept 3.1 Devices and Energy (A) · 3 lessons
2
Water that moves, energy that works
Discovery Education Science P4: Term 2, Unit 3, pp. 1–10
GoalExplain that moving water has kinetic energy and that people use it, and ask questions about dams.
New words
kinetic energy · the energy of anything that moveswatermill · a machine that uses moving water to turn a wheeldam · a wall built across a river to hold water back
Explanation
Water that flows down a river, falls from a height or rises in a wave is moving, so it has kinetic energy. The stronger the flow, the more energy it has. For hundreds of years people used this energy: a stream pushed the paddles of a watermill, and the turning wheel crushed grain into flour. Today the same idea is used to make electricity. By the end of this unit we will see how the energy of water can be turned into useful electricity, and how this changes the places around it. A dam is a wall that holds water back and then lets it flow through machines. Dams bring benefits, but they also change the land, the plants, the animals and the people nearby. Far away, in southern Africa, a very large dam called Kariba is a well-known example. Scientists and engineers ask: what are the good and the bad effects?
water flows or falls→it has kinetic energy→it turns a wheel or a turbine
Worked examples
In an old mill, a stream pushes the paddles of a wheel. The wheel turns and crushes grain into flour. The energy of the moving water does the work.
Stream
Distance
Time
Speed
fast
6 m
3 s
2 m/s
slow
6 m
6 s
1 m/s
Two streams: a slow one and a fast one. Youssef floats a leaf on each. The leaf on the fast stream travels 6 m in 3 s, which is 6 ÷ 3 = 2 m/s. The leaf on the slow stream travels 6 m in 6 s, which is 1 m/s. The fast stream has more kinetic energy.
Hana asks questions about a dam. How does a dam change the river below it? What happens to the fish? Where does the water go? Good questions can be investigated with books, videos and the advice of an engineer.
Class activity
Water wheel test. A grown-up helps you at a sink or a basin. Hold a plastic spoon, a paper cup or a pinwheel under a gentle stream of water and watch it move. Then use a faster stream. Which turns faster? Keep clothes dry and wipe up water so nobody slips. Write: moving water has energy because…
Practice
1
Why does a fast river have more kinetic energy than a slow one?
Its water moves faster, and faster moving things have more kinetic energy.
2
How did people use watermills before electricity?
Moving water turned a wheel that crushed grain into flour.
3
A leaf moves 12 m down a stream in 4 seconds. What is its speed?
12 ÷ 4 = 3 m/s.
4
What does a dam do? Write one good effect and one change it can cause.
It holds water back and lets it flow through machines. Good: electricity. Change: the land behind it is covered and fish habitats change.
