### Coefficient of Friction

### Definitions

- Define normal force, F
_{N}, as the force or the component of a force which a surface exerts on an object with which it is in contact, and which is perpendicular to the surface.
- Define frictional force, F
_{f}, as the force that opposes the motion of an object and which acts parallel to the surface.
- Define static frictional force, f
_{s} , as the force that opposes the tendency of motion of a stationary object relative to a surface.
- Define kinetic frictional force, f
_{k} , as the force that opposes the motion of a moving object relative to a surface.

Alternative Symbols
- Normal force is called N
- Friction Force is called f

### Know that a frictional force:

- Is proportional to the normal force
- Is independent of the area of contact
- Is independent of the velocity of motion

### Solve problems for static friction using:

where

is the maximum static frictional force and

is the coefficient of static friction.
- If a force, F, applied to a body parallel to the surface does not cause the object to move, F is equal in magnitude to the static frictional force.
- The static frictional force is a maximum just before the object starts to move across the surface.
- If the applied force exceeds the static frictional force, a resultant (net) force accelerates the object.

### Solve problems for kinetic friction using:

where

is the kinetic frictional force and

is the coefficient of kinetic friction.

### Examples

**1. Study the diagram which shows a force of 30N applied to a mass of 3kg.**

The coefficient of kinetic friction is 0,2.

**1.1. Draw a force diagram showing all the forces, with names.**

**1.2. Calculate the magnitude of the friction force.**

**1.3. Calculate the acceleration. **