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.