Reducing friction
Imagine that you have found an old bicycle. It has a black frame and black metal mudguards. The handlebars bend towards you when you sit on the saddle so you have to ride it sitting up. Some parts of the bicycle are chrome plated and still shine. You are lucky; the bicycle has been stored in a dry place so the metal has not rusted. When you wheel the bicycle out you find that its wheels turn a bit stiffly. When you climb on the well-sprung saddle and start to pedal, you find that the chain and gears crunch together as they move. Clearly friction is stopping you enjoying a ride on the old bicycle but help is at hand in the form of a can of oil.

You let the oil drip over the chain and the gears on the back wheel. You dribble the oil over the gear and chain by the pedals. You add oil to the front and rear axle and to the crankcase where the pedals meet. After you have turned the pedals a few times and moved the wheels backwards and forwards a few times the bicycle moves much more smoothly and you ride away.

What has happened to make the old bicycle a joy to ride? The oil has reduced friction where metal surfaces meet. A surface of a metal may look smooth to the naked eye but put it under a microscope and things look very different. The smooth surface looks rough. There are humps and hollows all over its surface. When two metals meet, the humps on one surface sink into the hollows in the other. If you try to push one surface past the other the humps and hollows try to stop you. While you are pushing gently the humps and hollows succeed. They create a force of friction, which pushes back against the force you are making and the pieces of metal stay still. When you push harder, the humps rise out of the hollows and each surface scrapes along the other one. There is still a force of friction between the surfaces but it is smaller than the one that held the surfaces together. When you pedalled your bicycle at first, it was this sliding friction which was making the bicycle difficult to move. The oil changed all that.

Metals are solids and oil is a liquid. Both are made from tiny particles, which can only be seen by the most powerful microscopes. The particles in a solid are all firmly stuck together like bricks in a wall but the particles in a liquid are different. They are not firmly attached together and can slide over each other like the grains in a packet of rice. When you oiled your bicycle, the oil coated the metal surfaces. Its particles went down into the hollows and filled them up. They coated the humps too. When the oiled metal surfaces of the gear and chain moved over each other the bumps could not get into the hollows. They bounced along the top. This is one way that the oil reduces friction but there is also a second way. The oil also coats the humps on both surfaces. When the humps bounce together, the oil particles between them slide over each other. This makes the humps slide or skid past each other and reduces the force of friction between the surfaces even further.

As you pedal away on the old bicycle, your speedy journey has been made possible by the movement of tiny particles of oil flowing over the humps and hollows of the moving metal parts.

Do other liquids have particles like oil?
Yes, they do. All liquids have particles that slide over each other. Some liquids, such as syrup, have particles which hold onto each other as they slide. This makes the liquid sticky. Other liquids, like water, have particles which do not hold on much as they slide. This makes the liquid runny.

Can water reduce friction?
Yes, it can. Think of walking carefully over a floor which has just been mopped. The particles of water fill the hollows in the surface of your shoe and the floor just as they fill the hollows in the metal surfaces. The water particles on the humps slide over each other too when the humps touch so water reduces friction like oil.

Why do we not use water to reduce friction on a bicycle?
The water does not cling as well to the metal as oil does, so there would soon be surfaces which were not covered. Water also evaporates much more quickly. This means that the metal surface would soon dry out. If water did collect in places on the metal when the bicycle was not in use, it could cause the metal to go rusty if it was made of steel.

What happens to the water when it evaporates?
The particles of water nearest the surface leave the water and go into the air. They form a gas called water vapour. Eventually all the particles move away into the air and form water vapour.

Why doesn't oil evaporate as quickly as water?
The oil particles hold onto each other more strongly so at any time far fewer oil particles escape into the air.

Why is oil put into the crankcase between the pedals?
Inside the crankcase is a ball race. This is made up of metal balls called ball bearings arranged in a circle. One way to think about how a ball race looks is to imagine marbles running in the groove around the top of a tin with a lid. When the ball race is oiled the ball bearings turn more easily. There is a ball race at the front of the bicycle where you turn the handle bars and front wheel. It helps you steer the bicycle more easily.

Do car engines need oil?
Yes, they do. We usually think of a car engine just needing petrol to keep it going but it needs oil too. The petrol provides energy for the car to work. Inside the engine the petrol is heated with air until it explodes. The petrol does not explode all at once - it explodes in small amounts inside cylinders in the engine. Inside each cylinder is a piston. When an explosion occurs, the piston is pushed down. It passes on this pushing force to the wheels and it turns the wheels. Once an explosion has taken place the piston moves back up the cylinder again ready for the next explosion. The pistons move up and down the cylinder hundreds of times a minute. This not only wears out the metal in the engine but also makes a great deal of heat. Oil is put into the engine to make the moving metal parts, such as the pistons, move more easily over each other. It also helps to reduce the amount of heat made by the engine.

Is the oil put in with the petrol?
No. Petrol is stored in a petrol tank, usually at the back of the car. The petrol is pumped through to the engine, as it is needed. The oil is poured directly into the engine. While an engine needs a tankful of petrol every few hundred miles, it may only need a small amount of oil every thousand miles or so. About every six thousand miles a car should have a service. This is done at a garage where all the parts are checked to see that they are working properly. When the car is having a service all the oil in the engine is drained out and new oil is put in. The change is made because oil, like metal can wear out too.