6.6:
Static and Kinetic Frictional Force
The magnitude of frictional force has two forms, namely static frictional force and kinetic frictional force.
The frictional forces can be explained using the applied force versus the frictional force graph. In the case of static frictional force, the applied force is exactly counterbalanced by the frictional force.
Hence, the net horizontal force is zero and the object remains at rest. Within the static frictional region, the graph is linear. Once the applied force exceeds the maximum value, the object accelerates and the frictional force decreases.
Now, the kinetic frictional force takes over and the object begins to move. Within the kinetic frictional region, it is easier to keep the object moving than to start moving it.
Hence, for any given pair of surfaces, the magnitude of the kinetic frictional force is always less than that of the maximum static frictional force.
6.6:
Static and Kinetic Frictional Force
One of the simpler characteristics of sliding friction is that it is parallel to the contact surfaces between systems, and is always in a direction that opposes the motion or attempted motion of the systems relative to each other. If two systems are in contact and moving relative to one another, then the friction between them is called kinetic friction. For example, kinetic friction slows a hockey puck sliding on ice.
However, if two systems are in contact and are stationary relative to one another, then the friction between them is called static friction. The static friction is usually greater than the kinetic friction between two objects. For example, if a person is trying to slide a heavy crate across a concrete floor, they may be pushing very hard but the crate does not move at all. This means that the static friction responds to what we do; it increases to be equal to the push and acts in the opposite direction. If the person pushes hard enough, and the crate starts to move, this is the moment when static friction gives way to kinetic friction. Once in motion, it is easier to keep an object in motion than to get the motion started, indicating that the kinetic frictional force is less than the static frictional force.
This text is adapted from Openstax, University Physics Volume 1, Section 6.2: Friction.