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 more difficult for things to move.

Helpful frictional forces

Friction can be useful. For example:

  • 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.

Air resistance

Bikes, cars and other moving objects 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.


Cyclist racing around velodrome.
Streamlining reduces air resistance

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

British Cycling coach Charlie Evans on how physics can help to give cyclists an edge