Question

What is meant by saying that the friction is a self-adusting force

Answer 1

Friction (like friction drag) is a contact force which results from two surfaces contacting each other. Static friction is considered a self adjusting force because it wants to the objects to remain at rest and not move. Therefore, if an external force is applied, the static friction force will equal the magnitude of the external force, until it surpasses the threshold of motion. At this point, static friction becomes kinetic friction and the object begins to move.
(Kinetic friction is not a self adjusting force)

Simply when a force is applied to a body within the limiting friction, the frictional force will be just equal to the force applied to remain rest.
Even if a small amount of force is increased or decreased within limiting friction, the force provided by the friction will still be the same as the present applied force only, so as to remain in rest. So it is called self adjusting force.

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