Friction and Stopping Distance

Friction is defined as a force that opposes motion.

When two objects are brought into contact the molecules from the surface of one object get very close to the molecules on the surface of the other object. This results in forces of attraction between the molecules and this must be overcome so that one surface can move over the other.

Objects moving through fluids such as air or water also encounter frictional forces which reduce their motion. This is known as drag.

Friction prevents objects from moving or slows them down. It also causes wear on surfaces as they rub against each other and generates heat. Thus energy is wasted in overcoming friction.

Reducing friction


Oil is used as a lubricant and provides a thin layer of liquid which separates the moving surfaces.


Ball bearings reduce frictions by making the moving surface roll rather than slide.


By designing the object to allow the easy flow of fluid around it (streamlining) as is done for rockets and racing cars the drag can be reduced considerably.

 Uses of friction

Although in a lot of cases friction is a hindrance but it is also very essential. In order to walk we depend on the friction between the soles of our feet or shoes and the ground. The tyres and brakes on vehicles depend on friction to stop, slow down and start moving. It is the air resistance or drag that slows down a parachute.

Stopping Distance

Cars rely on friction at the brakes and tyres in order to stop.

The total distance a car requires to stop is called the total stopping distance. This is the sum of the distance covered in the time it takes for the driver to react known as the thinking distance and the distance the car travels before coming to rest after the brakes are pressed called the braking distance.

Total stopping distance = thinking distance + braking distance

The diagram below gives the shortest stopping distances for a car on a dry road with good brakes and tyres.


From the diagram a clear pattern can be seen between the velocity of the car and the total stopping distance. The average reaction time of a driver is about 0.7s as the velocity of the car increases so will the thinking distance from the relationship;

Distance travelled = velocity x reaction time

The braking distances increases because at a faster velocity the car possesses more kinetic energy. This energy has to be transferred to the brakes; this is transferred as heat and is the reason why brakes become hot. The relationship between velocity and kinetic energy is,

Kinetic Energy (Joules) = ½ x mass x velocity2

Therefore if the car travels three times the velocity it has 9 times the kinetic energy which means the braking distance will be nine times longer.

 Factors affecting the total stopping distance

The driver’s reaction time

The thinking distance depends on the driver’s reaction time. Reactions are strongly influenced by the state of the mind. Thus, a driver under the influence of a drug such as alcohol will have a much reduced reaction time which will increase the thinking distance. Tiredness and fatigue also influence reaction time. A tired driver will react a lot slower than an alert driver.


As illustrated in the chart above the braking distance increases with velocity. This is explained by the relationship between kinetic energy and velocity. The greater the velocity the greater the kinetic energy the brakes have to transfer.


The mass of the vehicle is also related to kinetic energy in the relationship;

Kinetic Energy (Joules) = ½ x mass x velocity2

The greater the mass the greater the kinetic energy, thus a heavier car will require a longer braking distance.

Road Surface

Friction between the car tyres and the road surface stop it from skidding and sliding. On a wet or icy road surface the contact between the tyres and the road is considerably reduced. If the driver brakes hard the car will skid therefore the driver needs to apply a reduced force on the brakes increasing the braking distance.

Tyre Condition

Tyres are designed with grooves which channel the water away on wet roads in order to ensure contact of the tyre is made with the road. If the tyres are worn the friction between the road and tyres is reduced increasing braking distance.

Brake Condition

Worn brakes will take longer to transfer the kinetic energy of the car increasing the braking distance.