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Science

Forces

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Frictional forces

Whenever an object moves against another object, it feels frictional forces. These forces act in the opposite direction to the movement. Friction makes it harder for things to move.

Helpful frictional forces

Friction can be useful:

  • 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 slow down

Frictional forces are much smaller on smooth surfaces than on rough surfaces, which is why we slide on ice.

Unhelpful frictional forces

Friction can also be unhelpful. If you don't 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 lost to the surroundings as heat. Think of what happens when you rub your hands together quickly. The friction warms them up.

Air resistance

Bikes, cars and other vehicles experience air resistance as they move. Air resistance is caused by the frictional forces of the air against the vehicle. The faster the vehicle moves, the bigger the air resistance becomes. The top speed of a vehicle is reached when the force from the cyclist or engine is balanced by air resistance.

Streamlining

a streamlined cyclist

A streamlined racing cyclist

Racing cyclists crouch down low on their bikes to reduce the air resistance on them. This helps them to cycle faster. They also wear streamlined helmets. These have special, smooth shapes that allow the air to flow over the cyclist more easily.

Modern cars are also streamlined. Their smooth shapes make the air resistance smaller, which allows them to travel further on the same amount of fuel.

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