Forces that balance
Can you remember the first time you tried to ride a two-wheeler bicycle? The chances are that you wobbled all over the place and then fell off. If you were not too shocked at your failure you may have climbed on the bicycle again and had another go. Perhaps the second time you may have been more successful and moved the handlebars from side to side as you tried to stay upright and push down on the pedals. In time you probably found that the bicycle became much easier to ride and today you have no problem riding any bicycle you are allowed to. The reason you can ride a bicycle successfully today is because you have learned how to balance the forces acting on you when you sit on the saddle.

Imagine that you have just sat on a bicycle and are about to set off. What are the forces acting on you? First you will have one foot on the ground. Some of the weight of your body pushes down through your foot to the ground below. The ground pushes back with a force which is equal to this, so your foot does not sink into the road. At the two places where the wheels touch the ground the weight of the bicycle and some of your weight pushes down. These forces are also matched by equal forces from the ground which prevent the wheels from sinking.

When you start to move off the first thing that you do is raise your foot off the ground and place it on the pedal. This used to be the point at which you fell over when you were learning to ride a bicycle. You fell because when you raised your foot, that side of your body was leaning slightly over it. This meant that the force of gravity was pulling on more matter or mass on this side of your body than on the other side. The combination of mass and the force of gravity produces a force called weight so the weight pushing down on this side of your body was greater than on the weight pushing down on the other side and you fell over.

The next time you got on the bicycle you learned to push on the ground with your foot. This force tipped your body upright so that you could place your foot on the pedal without falling over. You have parts of your ears which are sensitive to balance and they send messages to your muscles to help you keep your body upright. The way these balance-sensitive parts and muscles work together is firmly established to help you walk and run but when you learn to ride a bicycle they have to learn to link together in a different way to keep you in the saddle. Fortunately these links do not usually take too long to become established and you soon learn to enjoy cycling.

As soon as both feet are on the pedals, the weight of the bicycle and your own body pushes down through the wheels and the ground pushes back. When you push down on a pedal, the force turns the gear wheel and pulls the chain. This in turn pulls on the gear at the axle of the back wheel and makes the back wheel turn. There is a force of friction between the tyre and the ground, which matches the way in which the wheel pushes backwards. This force pushes the bicycle forward and you are on your way.

When people first ride a bicycle many do not pedal again but let the bicycle freewheel and they enjoy the sensation of moving forwards. However they soon find that their journey does not take long as the bicycle slows down and stops. The reason for this is that other forces are acting on the rider and the bicycle. If the bicycle were out in space the one push on the pedal would be enough to keep it going in a straight line at a steady speed forever. The reason for this is that there are no forces to act on it in space. On Earth things are different. As the wheels run along the ground there is a force of friction acting on them. This slows the wheels down. As you move through the air you have to push it out of the way and the air pushes back with a force called air resistance. This force also slows you down. This means that the two forces acting together on the bicycle and rider soon bring a free-wheeling bicycle to a stop.

The way to keep a bicycle moving is to keep pedalling. When you do this, you apply a force to the bicycle, which is greater than the forces that slow you down, so you can travel as far as you like. Enjoy safe cycling.

Are forces balanced when you move?
Some forces are not balanced. The force that moves you forwards is greater than the forces that try to stop you. These forces are balanced when you are at rest. Your weight is balanced by the force pushing up from the ground whether you are at rest or moving. If they were not you might either rise up in the air as you went along the road or you would sink into it.

What happens when you raise your front wheel in the air and do a wheelie?
This is an unsafe thing to do. If a person does a wheelie they pull on the front of the bicycle with a force which is greater than the weight of the bicycle and this makes the front wheel rise into the air.

Why can you freewheel a long way down a hill?
You can do this because the force of gravity is pulling you down the side of the hill.

Why is it hard to pedal up a steep hill?
The force of gravity is pulling you backwards down the hill and you have to exert a force which will overcome this in addition to the forces of friction and air resistance.

At slow speeds are friction and air resistance equally important?
No. Air resistance increases as you go faster. You can feel the push of air resistance on your face. At slow speeds you will feel very little because air resistance is then low but if you cycle fast you will feel it push more strongly on your face.

Where does the force in your muscles come from?
It comes from the energy in your food. The food travels round your body in the blood. When it reaches the muscles the energy is released and is used to work the muscles. Inside the muscles are tiny strands. The muscles use the energy to make the tiny strands slide over each other. When they do this they make the muscles shorter and the muscles pull on your leg bones to exert a force on the pedals.

If we gain the energy from food why can cyclists become out of breath when they pedal hard?
The body needs oxygen as it releases energy from food. It gets the oxygen it needs from the air. Sometimes when you pedal hard you use up more energy than your body has oxygen for. When this happens, you get out of breath. You must then stop pedalling and breathe deeply so the oxygen supply can catch up with the energy your body is releasing.