Friction Study Material

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Friction

Introduction: If we project a block of mass m with initial velocity v0 along a long horizontal table, it eventually comes to rest. This means that, while it is moving, it experiences an average acceleration a that points in the direction opposite to its motion. If (in an inertial frame) we see that a body is being accelerated, we always associate a force, defined from Newton’s second law, with the motion. In this case we declare that the table exerts a force of friction, whose average value is ma , on the sliding block. Actually, whenever the surface of one body slides or has a tendency to slide over that of another, each body exerts a frictional force on the other, parallel to the surfaces. The frictional force on each body is in a direction opposite to its motion relative to the other body. Frictional forces automatically oppose the relative motion and never aid it. Even when there is no relative motion, frictional forces may exist between surfaces (but there must be a tendency of relative motion). Consider a block at rest on a horizontal table as shown in fig. 3.1. We find that the block will not move even though we apply a small force. We say that our applied force is balanced by an opposite frictional force exerted on the block by the table, acting along the surface of contact. As we increase the applied force we find that there is some definite value of theapplied force at which the block just begins to move. Once motion has started, this same force (without increasing any further) produces accelerated motion. By reducing the force once motion has started, we find that it is possible to keep the block in uniform motion without acceleration; this force may be small, but it is never zero.

F=0

fs

F No Motion; fs= F

fs

F

fs a

F

fk

F accelerated motion fk < F uniform motion fk = F

fk

a=0

F

v

Fig. 3.1

PHYSICS: Friction

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The frictional forces acting between surfaces at rest with...