 ### Definitions

1. Define normal force, FN, 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.
2. Define frictional force, Ff, as the force that opposes the motion of an object and which acts parallel to the surface.
3. Define static frictional force, fs , as the force that opposes the tendency of motion of a stationary object relative to a surface.
4. Define kinetic frictional force, fk , 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.
1. 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.
2. The static frictional force is a maximum just before the object starts to move across the surface.
3. 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. 