WHAT ARE THE LAWS OF FRICTION

Friction is actually a force that appears whenever two things rub against each other.

LAWS OF FRICTION :

Law 1
When two bodies are in contact the direction of the forces of Friction on one of them at it's point of contact, is opposite to the the direction in which the point of contact tends to move relative to the other.
Law 2
If the bodies are in equilibrium, the force of Friction is just sufficient to prevent friction and may therefore be determined by applying the conditions of equilibrium of all the forces acting on the body.

The amount of Friction that can be exerted between two surfaces is limited and if the forces acting on the body are made sufficiently great, motion will occur. Hence, we define limiting friction as the friction which is exerted when equilibrium is on the point of being broken by one body sliding on another. The magnitude of limiting friction is given by the following three laws.
Law 3
The ratio of the limiting friction to the Normal reaction between two surfaces depends on the substances of which the surfaces are composed and not on the magnitude of the Normal reaction.

This ratio is usually denoted by $mu$ .
Thus if the Normal reaction is R, the limiting friction is $mu:R$
For given materials polished to the same standard $mu$ is found to be constant and independent of R.
$mu$ is called The Coefficient of friction
Law 4
The amount of limiting friction is independent of the area of contact between the two surfaces and of the shape of the surfaces, provided that the Normal reaction is unaltered.
Law 5
When motion takes place the direction of friction is opposite to the direction of relative motion and independent of velocity. The magnitude of the force of friction is in a constant ratio to the Normal reaction but this ratio may be slightly less than when the body is just on the point of moving.

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Laws of friction. We know from experience that when two bodies tend to

slide on each other a resisting force appears at their surface of contact which

opposes their relative motion. The laws of dry friction are best understood by the

following experiment. A block of weight W is placed on a horizontal surface

(Fig.4.1a). Suppose, now, that a horizontal force P is applied to the block

(Fig.4.1b). If P is small, the block will not move; some other horizontal force

must therefore exist, which balances P. This other force is the static-friction

force F, which is actually the resultant of a great number of forces acting over

the entire surface of contact between bodies. The nature of these forces is not

known exactly, but it is generally assumed that these forces are due to the

irregularities of the surface in contact and also, to a certain extent, to molecular

attraction. A detail examination of the nature of friction is complex physicomechanical problem lying beyond the scope of theoretical mechanics.

If the force P is increased, the friction force F also increases, continuing to

oppose P, until its magnitude reaches a certain maximum value Fm (Fig.4.1c). If

P is further increased, the friction force cannot balance it any more and the

block starts sliding. As soon as the block has been set in motion, the magnitude

of F drops from Fm to a lower value Fk . From then on, the block keeps sliding

with increasing velocity while the friction force, denoted by Fk and called

kinetic-friction force, remains approximately constant.

Experimental evidence shows that the maximum value of static-friction force

Fm is proportional to the normal component N of the reaction of the surface, i.e.

Fm = µsN (4.1)

where µs

is a constant called the coefficient of static friction.

Similarly, the magnitude Fk of the kinetic-friction force may be put in the form

Fk = µkN (4.2)

where µk is a constant called the coefficient of kinetic friction.The coefficients of friction µs

and µk do not depend upon the area of the surfaces

in contact. Both coefficients, however, depend strongly on the nature of the

surfaces in contact. Since they also depend upon the exact condition of the

surfaces, their value is seldom known with an accuracy greater than 5%.

Approximate values of coefficients of static friction are given in Table 4.1 for

various dry surfaces. The corresponding values of the coefficient of kinetic

friction would be about 25% smaller.

Table 4.1 Approximate values of coefficients of static friction for dry surfaces

Metal on metal 0,15 ÷ 0,35

Metal on wood 0,20 ÷ 0,60

Metal on leather 0,30 ÷ 0,60

Wood on wood 0,30 ÷ 0,70

Rubber on concrete 0,60 ÷ 0,90

Steel on ice ~0,03

From the description given above, it appears that four different situations may

occur when a rigid body is in contact with a horizontal surface:

a) The forces applied to the body do not tend to move it along the surface of

contact; there is no friction force (Fig.4.2a);

b) The applied forces tend to move the body along the surface of contact but are

not large enough to set it in motion. The friction force F which has developed

may be found by solving the equations of equilibrium for the body. Since there

is no evidence that the maximum value of static-friction force has been reached,

the equation (4.1) cannot be used to determine the friction force F (Fig.4.2b);

If the magnitude F of the friction force is smaller than its maximum value Fm ,

i.e. F < Fm = µsN, we say that the friction is not developed

Fric-2c) The applied forces are such that the body is just about to slide. We say th