Always acts in the direction opposite to motion or possible motion.

Friction has a limit which it cannot exceed.  This is called limiting friction and is proportional to the normal contact force acting on a object.

The limiting friction depends on the object and the surface it is on and is governed by the equation F=μR, where μ is the coefficient of friction for the specific combination of object and surface.

If object is on the point of moving we say it is in limiting equilibrium.

μ is typically between 0.3 and 0.9, however it can be greater than 1 (it is an experimentally observed approximation and does not represent a mathematical law like Newton’s laws).

Surfaces in contact
Wood sliding on wood	0.2 – 0.6
Metal sliding on metal	0.15 – 0.3
Normal tyres on dry road	0.8
Racing tyres on dry road	1.0
Sandpaper on sandpaper	2.0
Skis on snow	0.02

Worked Examples

Exercise (skip questions 3 and 4 which are about total contact force. Total contact force is the resultant of the friction and the reaction force, which we haven’t talked about (and won’t;))

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Worked solutions

Limiting Equilibrium

F ≤ μR

It is only when an object is at the point of moving (in limiting equilibrium), that F = μR.

Worked Examples

Exercise

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Worked solutions

More Advanced Friction Questions

Worked Example

Exercise (and General Exercises)

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Worked solutions for both exercises