Friction
is the force between two surfaces rubbing together.
When
two smooth surfaces rub together there is very little friction.
When
two rough surfaces rub together there is more friction.
There
is less friction when there is a liquid (e.g. oil) between the two
surfaces. This is because liquid particles easily move past each other
There
is more friction if the two surfaces are forced against each other.
Friction makes things slow down
When we are on Earth - or any other planet for that matter- objects, once pushed, experience friction of some sort and therefore slow down. This made Aristotle wrongly believe that movement was evidence of a force acting (as once you stopped pushing the thing slowed down!). This is not the case! It is the invisible force of friction that acts to oppose the motion and confuse us. Acceleration is evidence of a force acting NOT movement. See Newton's Laws of Motion.
Friction can be a good thing:
Just
think of walking on a wet marble floor or an icy pavement with new leather
soled shoes: you might slip over! Worse still, if the tyres on your
car are worn out and the road is wet and slippery, you would probably
skid in the car and have an accident. Both your shoes and the car's
tyres need good grip so their surfaces are rough. This increases the
friction between them and the ground.
Friction
is also very important for your car's brakes to work properly. When
you put you foot on the brake pedal, some rough pads are squeezed tight
against the brake discs. This friction slows the car down. If oil gets
on the discs, the brakes will not work so well.
If the tyres are worn or the roads are wet braking can be a problem.
Friction
can be a bad thing:
Friction
inside a car engine and inside the wheel axles will slow a car down
and wear out the metal. To prevent this we put oil or even grease in
them. This makes their surfaces more slippery and so reduces friction.
Friction
in an engine causes a change of useful kinetic energy into heat energy.
This is wasteful. A smooth running engine will be more efficient...
because it will produce less unwanted heat energy and more of the type
we want. Oiling a machine and keeping its it clean can make it more
efficient.
NB
Friction is a force NOT a type of energy!!
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