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  Friction, Physics tutorial

Laws of Friction:

Frictional forces act along the surface between two bodies when one attempts to move or succeeds in moving over other. So Friction is the contact force. It is that force which attempts to or opposes motion. Rubbing surfaces in machinery require to be lubricated to decrease friction so that their life span could be extended. Yet we require friction as it allows us to walk without slipping. It allows us to keep things in standing positions. Wherever there is friction, you expect some surface wear of materials in contact.

Coefficient of Friction:

Coefficient of friction is also known as Frictional Coefficient.

It is denoted by the Greek letter U.

It is the scalar value with no dimensions.

It basically is ratio of frictional force acting between two bodies and force which is pressing them together.

Different materials have different frictional coefficient. It is symbolized by different subscripts like:

For Kinetic Friction: Uk

For Static Friction: Us

For rolling Friction: Crr

Basic equation of friction is, Ff = U Fn

Hence U = Ff/Fn

For static friction: Us = Fs/Fn

For kinetic friction: Uk = Fk/Fn

For rolling friction: Crr = F/N

Coefficient of Friction Formula:

Friction Coefficient (U) between two surfaces is stated as ratio of force which maintains contact between the object and surface (N) and frictional force (F) which resists motion of the object.

Equation for coefficient of friction is same as mentioned above:

U = FN . . . . . . . . . . . . . . where,

F = Force applied

U = Coefficient of friction

N = Normal force

These are series of steps involved in determining Coefficient of friction:

For static friction Fs = UsN

Hence Us= Fs/N

The body which overcomes static friction begins moving, and force then changes to Fk.

Like the case of kinetic friction,

Uk = Fk/N

If all values are given, just substitute to determine coefficients. Or follow the steps given below:

1) Determine Fs, Fk and N.

2) By Newton's Second Law of Motion, F = ma

Determine net force that is the sum of all forces acting on object. For zero acceleration net force will also be zero.

Nature of Friction:

Coefficients of static and kinetic (dynamic or sliding) friction rely on nature of surfaces in contact between two bodies. Coefficient of friction is large for rough surfaces than for smooth ones. Coefficient of kinetic friction differs with relative velocity but for simplicity we suppose it to be independent of velocity. Close examination of flattest and most polished surfaces reveals that there still exist hollows and humps which are more than one hundred atoms stacked one on top of other. This signifies that when two solid surfaces are placed one on other, or are made to touch, their real area of contact is very small. Electrical resistance measurements of two metals in contact tell that true area of contact between surfaces is very small. It is evaluated that in case of steel, actual area which is touching several be just approx one ten thousandth (1/10,000th) of the evident area really placed together. Two metal surfaces therefore sit on each other's projections when they are placed one on top of other. This goes for non-metallic objects as well

Pressures at points of contact between two metals are very high and cause bumps to flatten until increased area of content allows upper solid to be supported. It is supposed that at point of contact small, cold welded joints' are formed by strong adhesive forces between molecules which are very close together. These have to be broken previous to one surface can slide over other. This occurrence accounts for first law of frictional force.

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