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Dynamic and Static Friction

Friction opposes the relative motion of two surfaces in contact. It occurs because surfaces are not smooth on the microscopic scale. If the surfaces are not moving relative too each other, then there is static friction. If they are in relative motion, there is dynamic or kinetic friction. Static friction Fr is proportional to the reaction force from the surface N, up to a limit with the constant of proportionality being

We can write

Once the object starts to move friction reduces slightly as shown below.

The coefficient of friction depends on the two surfaces. Some are given below.

Surfaces

µ (static)

µ (kinetic)

Steel on steel

0.74

0.57

Glass on glass

0.94

0.4

Metal on Metal (lubricated)

0.15

0.06

Ice on ice

0.1

0.03

Teflon on Teflon

0.04

0.04

Tire on concrete

1

0.8

Tire on wet road

0.6

0.4

Tire on snow

0.3

0.2

Examples: There is friction in the bearings of engines and wheels. The friction can be reduced with the use of lubricating oil but it cannot be eliminated.