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=*μ*R, 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).

**Worked Examples**

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

**Answers**

**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**

**Answers**

**More Advanced Friction Questions**

**Worked Example**

**Exercise** **(and General Exercises)**

**Answers**