P2aL5 Stop!

Key Words

Braking distance - distance covered by a car when the brakes are on.

Directly proportional - a graph goes up in a straight line from the origin.

Kinetic Energy - energy of a moving object.

Reaction time - time taken for a driver to see an emergency, and apply his foot on the brake.

Resultant force - force that occurs on an object when two or more forces are applied.

Stopping distance - the total distance from when a driver sees an emergency to the car coming to a stop.

Thinking distance - the distance covered by the car as the driver is reacting to an emergency.

Grade E

When the brakes are applied on a car, there is a resultant force opposite to the direction of movement, so the car slows down.  Also the engine is driven by the movement of the car when the driver takes his foot of the accelerator.  A force is needed to do this, and it provides a backwards force.

In an emergency, there are two factors to consider:

• the driver's reaction time

• the distance needed for the car to stop.

While the driver is reacting, the car is travelling at a constant speed.  This is called the thinking distance.  Then, as the brakes are applied, the car decelerates.  This is the braking distance.  The stopping distance is these two added together.

Stopping distance = thinking distance + braking distance

Grade C

The thinking distance can be worked out by:

Thinking distance (m) = speed (m/s) × reaction time (s)

The thinking distance is directly proportional to the speed.  Double the speed, you get double the thinking distance.

The braking distance is determined by the kinetic energy of the car.  In this case, if you double the speed, the kinetic energy goes up, not twice, but four times.  So the braking distance goes up four times.

We can show the motion in a speed-time graph:

The distance is the area under the graph.  If we look at the thinking distance, the area of the rectangle for 40 m/s is twice what it is for 20 m/s.  However the area of the 40 m/s triangle is four times the area of the 20 m/s triangle.

Reaction time is increased by:

• Driving while unfit because of drink or drugs;

• Distraction like fiddling with the radio, or using a mobile telephone.

Braking distance is increased by factors like:

• The mechanical condition of the vehicle (i.e. the brakes aren't that good);

• The state of the tyres;

• The state of the road - is it icy, or wet?

Grade A

On a dry road, bald tyres reduce the stopping distance.  This is because there is more area in contact with the road.  Racing drivers use bald tyres (slicks) in dry weather.

On a wet road, a thin layer of water builds up under a bald tyre, which makes it aquaplane.  Therefore it can be impossible to control the car as there is almost no friction.  To drive on bald tyres is a serious motoring offence.

Racing drivers change tyres if it's going to rain.