The coefficient of friction is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together.

The coefficient of friction between the tires of a car and the road varies depending on the road conditions. This affects the stopping distance of the car.

On a dry road, the coefficient of friction between the tires and the road is high as the two bodies do not slide past each other easily. In this case the coefficient of friction would be about 0.7, and can get as high as 0.9. For wet roads it would be about 0.4, and can get as low as 0.1. The coefficient of friction is lower, and will increase the stopping distance of the car. On a dry road, the stopping distance of a car would be far less rather than that of being on a wet road. When the road is wet, the water lubricates the road and dramatically lessens the coefficient of friction, thus the stopping distance of a car in motion would be greater. On icy roads, the ice makes the road slippery and results in a greater stopping distance of a moving vehicle. It takes at least twice the distance for the vehicle to stop when the road is wet or icy compared to stopping on a dry road.

The equation for finding the coefficient of friction is:

u = Fr/Fn

Where:

u = coefficient of friction

Fr = force required to move the object

Fn = force normal, or the perpendicular weight of an object acting on a surface

The coefficient of friction varies with acceleration. Fr = ma, where m = the weight of the object, and a = acceleration. Therefore, the equation for finding the coefficient of friction can also be written as

u = (m x a)/Fn

The figures m and Fn are constant values, and a is the only variable. Due to this, if the acceleration has a higher value, the coefficient of friction is higher, and if the acceleration has a lower value, the coefficient of friction is lower. On wet roads, the acceleration is lower, and so the coefficient of friction will be lower. The acceleration is equal to the deceleration, hence making the deceleration lower and increasing the stopping distance of moving vehicle. Icy roads are more slippery than wet roads, so the acceleration and deceleration on a wet road would be even lower and further increasing the stopping distance of a moving vehicle.