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Friction is a force that opposes motion.

It is present whenever two surfaces make contact, such as when you rub your hands together, or when you apply the brakes on a bike or in a car.

Friction also prevents an object from starting to move, such as a shoe placed on a ramp.

When friction acts between two surfaces that are moving over each other, some kinetic (movement) energy is transformed (changed) into heat energy.

Helpful frictional forces

Examples of useful fricitional forces:

• friction between our shoes and the floor stop us from slipping
• friction between tyres and the road stop cars from skidding
• friction between the brakes and wheel help bikes and cars to slow down

Frictional forces are much smaller on smooth surfaces than on rough surfaces, which is why we slide on ice but not on concrete.

Unhelpful frictional forces

Friction can also be unhelpful. If you do not lubricate your bike regularly with oil, the friction in the chain and axles increases. Your bike will be noisy and difficult to pedal.

When there is a lot of friction between moving parts, energy is transferred to the surroundings, causing heating. Think of what happens when you rub your hands together quickly. The friction warms them up.

We try to reduce friction using bearings and lubricants.

Small ball shaped bearings reduce the size of the surface areas that are in contact. This help parts move easier against each other and spin with less resistence because the friction has been reduced. Oil lubrication, like on a bike chain, also reduces friction and both help to improve efficiency by reducing friction.