We need energy.
We should only use the amount of energy we need.
There are two serious problems when we waste energy:
Pollution. When we burn fuels, they produce carbon dioxide. This goes into the atmosphere and increases the temperature of our planet. This is called global warming.
Depletion of fossil fuels. These fuels are not renewable.
Coal, petroleum and natural gas are fossil fuels.
We extract coal from mines and petroleum from oil fields.
Natural gas is found in deposits deep beneath the surface of the Earth.
Petroleum is used to produce fuels, plastics and other products.
Natural gas and coal are used directly as fuels to produce electricity.
We use many types of energy from different energy sources.
There are two main types:
Renewable energy sources. These sources never run out, for example, sunlight and wind, or they are continually produced in nature, for example, wood.
Non-renewable energy sources. Examples are uranium and petroleum. These sources will run out one day because we use them up and they cannot be replaced.
Normally, forces act when two objects come into contact.
Sometimes forces act when there is no contact between objects.
This happens with magnetic force.
A magnet can attract or repel objects from a distance.
Magnets have two ends, or poles.
When we put the same poles of two magnets together, they repel each other.
When we put the different poles of two magnets together, they attract.
Force of attraction: A magnet pulls iron objects towards it.
Force of repulsion: Repulsion means when things push apart from each other.
Forces make objects start moving.
They also make them stop moving.
To move a football, we put force on the ball with our foot.
If nothing stops the ball, it continues to move.
However, it moves more and more slowly until it stops.
This is because a force on the ground acts on the ball and slows it down.
This force is called friction.
Soil is essential for people.
This is because plants need soil to grow.
We need plants for food and for wood.
Plant roots help fix soil and prevent wind and water sweeping it away.
Plant leaves and branches protect soil from heavy rains.
For this reason, we must protect the plants that grow in soil.
Soil is a mixture of many things.
It forms very slowly over a long period of time.
First, wind, rain and ice wear down rocks into smaller pieces.
Then, the action of animals and plant roots breaks the rocks into even smaller pieces.
Little by little, the pieces of rock mix with the remains of plants or animals.
These remains of living things turn into a dark mixture called humus.
The little pieces of rock mix with the humus to form soil.
There are different types of soils because they are formed from different mixtures of rocks and organic matter.
In many places, rocks are covered by a layer of earth, called soil.
Soil consists of stones, water, air, minerals and the remains of animals and plants.
Minerals are natural, solid substances.
All minerals are made up of only one substance.
They can be found as parts of rocks or in their pure state. Graphite: It is a natural substance.
It is a mineral.
It is soft and when we press it, layers slide off and leave marks on the paper.
Rocks can be found near the surface of the Earth or deep beneath the ground.
Mines. These are places where rocks and minerals are extracted from deep beneath the ground. Miners reach the rocks by travelling through shafts and tunnels.
Quarries. These are places where rocks are extracted from near the surface of the Earth.
Rocks form the solid part of the Earth.
They are found everywhere in nature.
Some rocks are dark (basalt).
Others are light in colour (limestone).
Some rocks are formed by large grains (granite).
Others are formed by fine grains (clay).
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