Whenever a body slides along another body, a resisting force is called into play which is known as the force of friction.

FRICTION

Whenever a body slides along another body, a resisting force is called into play which is known as the force of friction.

This is a very important force and serves many useful purposes, for a person could not walk without it, or a car could not propel itself along the road without the friction between the tires and the road. On the other hand, friction is very wasteful because it reduces the efficiency of machines, since work must be done to overcome it, and this energy is wasted as heat. The purpose of this experiment is to study the laws of sliding friction and to determine the coefficient of friction between two surfaces.

Friction is the resisting force encountered when one tries to slide one surface over another; this force acts along the tangent to the surface in contact. The force necessary to overcome friction depends on the nature of the materials in contact, their roughness or smoothness, and on the normal force, but not on the area of contact, within wide limits. It is found experimentally that the force of friction is directly proportional to the normal force. The constant of proportionality is called the coefficient of friction.

The coefficient of friction is equal to the force of friction divided by the total normal force pressing the surfaces together. Thus or where F is

the force of friction to be overcome, N is the total normal force, or the perpendicular component of the force holding the two surfaces together, and K is the coefficient of friction.

To determine the above relation the substances must be in the form of a plane placed horizontally, with a pulley fastened at one end. The other substance is made in the form of a block to which a cord passing over the pulley and carrying weights is attached; these may be varied until the block moves uniformly when given a very slight push. The normal force between the two surfaces can be changed by placing weights on top of the block, and the relation between the coefficient of friction, the force of friction, and the normal force can thus be tested.

The limiting angle of repose is the angle at which a body will just begin to slide down an inclined plane. The coefficient of friction is equal to the tangent of the angle of repose. It is found that the frictional force acting when actual sliding is taking place, is slightly lower than the maximum frictional force that can act just before the body begins to slide. Thus the kinetic coefficient of friction is somewhat lower than the static coefficient of friction.

in contact дотичні

is called into play вступає в силу, проявляється

necessary to overcome необхідна, для того щоб подоляти

when given a very slight push якщо їх легенько підшовхнути

Example. Friction: static and kinetic. Our 10 kg mystery box rests on horizontal floor. The coefficient of static friction is =0.40 and the coefficient of kinetic is =0.30. Determine the force of friction, , acting on the box if a horizontal external applied force is exerted on it of magnitude: (a) 0, (b) 10N, (c) 20N, (d) 38N, and (e) 40N.

Fig 1.3. Resting the box on horizontal plane.

When an object is pulled by is pulled force () along a surface, the force of friction opposes the motion. The magnitude of is proportional to the magnitude of the normal force ().

Solution. The free-body diagram of the box is shown in Fig.1.3. Examine it carefully. In the vertical direction there is no motion, so yields - . Hence the normal force for all cases is:

(a) Since no force is applied in this first case, the box doesn’t move, and .

(b) The force of static friction will oppose any applied force up to a maximum of

The applied force is . Thus the box will not move; since: the .

(c) An applied force of 20N is also not sufficient to move the box. Thus to balance the applied force.

(d) The applied force of 38N is still not quite large enough to move the box; so the friction force has now increased to 38N to keep the box at rest.

(e) A force of 40N will the box moving since it exceeds the maximum force of static friction, . Instead of static friction, we now have kinetic friction, and its magnitude is

There is now a net (horizontal) force on the box of magnitude , so the box will accelerate at a rate as long as the applied force is 40N. Figure 1.4. shows a graph that summarizes this example.

Fig 1.4. Friction force versus applied force.

Magnitude of the force of friction as a function of the external force applied to a body initially at rest. As the applied force is increased in magnitude, the force of static friction increases linearly to just match it, until the applied force equals . If the applied force increases further, the body will begin to move, and the friction force drops to a roughly constant value characteristic of kinetic friction.

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