Stopping Distance

The exam board appears to be very keen on aspects of physics that relate to real life – and vehicle stopping distance is an excellent example. You should therefore ensure that you are confident about this topic as there are standard “right” answers that you need to know.

Stopping distances are divided into two parts. First there is the thinking (or reaction) distance, which is the distance the vehicle will travel while the driver is reacting to a situation before the brakes are applied. Then comes the braking distance, which is the distance the vehicle continues to travel while the brakes are applied before it comes to a halt.

The thinking distance will be longer than usual if the driver is tired, distracted or under the influence of drink or drugs. This is because these things generally give people slower reaction times, so clearly the vehicle will move further before the driver has time to react and press the brake pedal.

The braking distance will be longer than usual if there is some sort of mechanical problem that results in less grip between the tyres and the road. That could be because the tyres are worn or because the road is slippery (wet, icy or covered in leaves or oil). It could also be because the brakes are worn.

From a physics perspective, there are also another two factors that increase the braking distance: these two factors are the mass of the vehicle and its velocity (speed). Importantly, these two factors have very different effects. If the mass of a vehicle is doubled then the braking distance will be doubled. But if the speed of the vehicle is doubled, the braking distance will be quadrupled.

To explain why this is, we need to look carefully at the kinetic energy equation, which will be covered in a separate article.