Pedal power
Riding a bicycle is a perfect way to find out about force. When you are moving at a steady speed the force produced by your legs exactly balances friction, the force that tries to slow you down. You can never get rid of friction, but you can reduce it by keeping the bike oiled and your head down so that you lessen the friction caused by the air you push of the way.

1. Going up
When you go uphill, you have to counteract the downward force of gravity. The bike starts to slow down unless you put in extra work to match the downward pull.

2. Going down
Going downhill is easy, because this time gravity is on your side. If you freewheel and keep your hands off the brakes, the bike starts to speed up. It keeps speeding up until friction matches the pull of gravity.

3. On the level
On flat ground, gravity has no overall effect. To keep going at a steady speed, all you have to do is match the force of friction. Some of the friction is produced by the moving parts of the bike. The rest of it is produced by the air you push out of the way. If you go really fast, 'air resistance gets very large.

4. Time to Stop
As soon as you stop pedalling, friction and air resistance gradually bring the bike to a halt. Another form of friction, jamming on the brakes, stops the bike much more quickly.

Staying on course
Once something is on the move, it will keep moving a straight line. This is why passengers are thrown forwards when a car brakes. It is also why water stays in a bucket if you whirl it around in a circle, while the bucket turn a corner, the water tries to continue straight on.

Strapped in
Seat belts bring people to a safe halt if a car stops suddenly.

How to lose weight
Because the Earth is not exactly round, the force of gravity varies slightly fro place to place. If you weight 30 kg (66 lb) at the North Pole, you would find that you weighed only 29.85 kg (65.8 lb) at the Equator.

FORCE AND MOTION

Force are at work all around us. They provide the push that gets things moving, but also include friction, the force that slows them down. They can make a jumbo jet hurtle down a runway and into the skies, or pull a snowflake towards the ground. Forces can also make things change direction, and they can stretch, squash and bend them out of shape. We cannot see forces themselves, but we can see and feel what they do.

How does gravity work?
We know what gravity feels like, but it is still a mysterious force.Everything in the Universe, even your own body, attracts other objects toward it. When you stand on the ground, the Earth attracts your body and your body attract the Earth. Because the Earth is so huge, its gravitational pull keeps you on the ground.

Copper
A reddish metal, is one of the few elements that is sometimes found in a pure form, rather than being mixed with other elements in a compound.

Carbon atom
This is a single atom of carbon. In the middle, it has a nucleus made of six neutrons. Six tiny particles called electrons spin around them. Carbon atoms are good at making molecules, because they can link up with lots of other atoms. As well as plastics, they are found in thousands of other substance, including most of the ones in living things.

Atoms and elements
Most substance contain a mixture of different atoms. Element are different, because they contain one type of atom and nothing else. Most elements are metals but some are gases, such as oxygen, and others are nonmetals, such as sulfur. Elements generally combine with other elements, making chemical compounds, such as water, sugar and salt. Altogether, 92 elements exist naturally on Earth, but scientists have made about 20 additional elements in laboratories.