For Objects at Rest: Static Friction

• The coefficient of static friction depends upon the roughness of the surfaces.

• The coefficient of static friction does not depend upon contact surface area, provided the two surfaces are made of dissimilar enough material so that any cohesive force between the two surfaces is small.

• The force of static friction (like the normal force) is a reaction force in that its value depends upon the magnitude of some external force trying to push the object along the surface.

• The force of friction is always oppsite to direction of the other forces imposed.

• The force of static friction increases up to a maximum value, after which the object "breaks loose" and begins to start moving.

For Objects in Motion: Kinetic Friction

• The coefficient of kinetic friction (like the static coefficient) depends upon the roughness of the surfaces. However it is generally smaller in value than the static coefficient.

• For relative velocities in the range of cm/s to m/s (typical ranges we will encounter) the coefficient of kinetic friction is approximately constant.

Force of Friction on a Block
An horizontal force is applied to a block resting on a rough surface. Displayed are the various forces acting on the block as it move along the surface. The mass of the block, the applied force, and the coefficient of friction can be altered. Force of Friction on a Block QT Movie

Block on an Incline Plane
A block is pushed up an incline plane by an force parallel to the incline plane. The coefficient of friction between the block and the incline plane, the applied force, the angle of the incline plane, and the mass of the block can be varied. Displayed are the resulting forces acting on the block as it moves up or down the incline plane. Block on Incline Plane QT Movie