# Kinetic friction

## Question:

Why does friction acting on a body decrease with increase in speed? It is said that FRICTION is independent of area of contact then why is that the radial tyres are better than ordinary tyres?

Friction is only independent of area of contact between two metals because the two surfaces are not flat and only touch at a few points where tiny projections on the two surfaces meet (and cold weld). With a tyre the radial tyres are generally thought to be better because they are stronger because of the way in which they are constructed. The tread on tyres removes water from the tyre in the wet to prevent the tyre "planing" on the water.

As regards kinetic friction.
The equations usually assume a constant of kinetic friction that is independent of the velocity – and not as you suggest.

With cars braking to a stop the frictional forces in the brakes are assumed to be the same whatever the speed of the car. This enables stopping distances to be worked out. However it is a bit more complicated than that. If we assume that the car does not skid there is no relative motion between the tyres and the road and so we can use the static friction coefficient here.

However the frictional drag of an object falling through a fluid does depend on the velocity of the object. The drag increases up to a point where the drag is equal to the weight of the object and the object then falls at its terminal velocity. The drag force is usually taken as proportional to velocity squared.

At low speeds the kinetic friction is a stick-slip motion as the "welds" between the surfaces are constantly broken and then reform. At higher speeds there is not time for the welds to reform and this might suggest the lowering of the friction at higher relative speeds.

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