Static frictional forces from the interlocking of the irregularities of two surfaces will increase to prevent any relative motion up until some limit where motion occurs. It is that threshold of motion which is characterized by the coefficient of static friction. The coefficient of static friction is typically larger than the coefficient of kinetic friction.
When we move forward the entity located on table with little force, the entity does not shift at all. It means that surface of contact of entity and table applies some force in a direction opposed to that of push.
Thus when block be inclined to shift on surface of table, a force start to act which opposes the motion of block on the table. This opposing force is known as force of friction or frictional force or friction.
When two surfaces are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds, and in the standard model of friction the frictional force is described by the relationship below. The coefficient is typically less than the coefficient of static friction, reflecting the common experience that it is easier to keep something in motion across a horizontal surface than to start it in motion from rest.
Kinetic friction is also known as sliding friction. When external force exceeds the limiting friction, the body begins to move on the surface. Then the force of friction is called the kinetic friction.
When an external force is applied to start relative motion, the interlocking of surfaces gets teared off, thus causing less frictional force. That is why once the motion starts, the frictional force acting between the surfaces in contact decreases, so that a smaller force is required to maintain uniform motion. That is why kinetic friction is less than the static friction.
Kinetic friction is constant whatever the velocity of body may be and is always less than the limiting friction.
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