Definitions

1. 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.
2. Define frictional force, F_f, as the force that opposes the motion of an object and which acts parallel to the surface.
3. Define static frictional force, f_s , as the force that opposes the tendency of motion of a stationary object relative to a surface.
4. Define kinetic frictional force, f_k , as the force that opposes the motion of a moving object relative to a surface.

• 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:

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:

Examples