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Science

Weight and friction

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Stopping distances

You should know some of the factors affecting the stopping distance of a car.

Thinking distance

It takes a certain amount of time for a driver to react to a hazard and start applying the brakes. During this time, the car is still moving. The faster the car is travelling, the greater this thinking distance will be.

The thinking distance will also increase if the driver's reactions are slower because they are:

  • under the influence of alcohol
  • under the influence of drugs
  • tired

Braking distance

The braking distance is the distance the car travels from where the brakes are first applied to where the car stops. If the braking force is too great, the tyres may not grip the road sufficiently and the car may skid. The faster the car is travelling, the greater the braking distance will be.

The braking distance will also increase if:

  • The brakes or tyres are worn.
  • The weather conditions are poor, such as an icy or wet road.
  • The car is more heavily laden, for example, with passengers and luggage.

Stopping distance

The stopping distance is the thinking distance added to the braking distance. The graph shows some typical stopping distances.

a car moving at a  speed of 20 miles an hour would have a thinking distance of 7 miles, and a braking distance of 7 miles. A car moving at 70 miles per hour would have a thinking distance of 22 miles, and a braking distance of nearly 80 miles.

Stopping distances

Watch this video to see a summary of how gravity and friction forces affect motion.

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