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Friction

Nature of Friction. — When a heavy block of wood is pushed along the top of a table, a certain resistance is encountered. By making the surface of the table and the surface of the block very smooth, the amount of this resistance can be much decreased. No matter what the nature of the surfaces that are moving over each other, there is al­ways some resistance, or opposition, to the motion. This resistance, the amount of which depends in part on the characteristics of the rubbing surfaces, is called friction. It always opposes the motion, what­ever its direction. It never tends to push the body either forward or backward. It merely tends to stop the motion or to make it more difficult to move the body. It is more difficult to start the body than it is to keep it in uniform motion when once started. There are for this reason two kinds of friction, kinetic friction and static friction. The former is the force to keep the body moving with uniform speed. The latter is the force to start the body from rest. Kinetic friction is less than static friction.

Sliding friction means the force of opposition offered to the sliding of one surface over another. The force is due to the minute irregularities of one surface engaging in those of the other. A simple example оf the resistance which a body encounters is when it slides or tends to slide over another.

Friction between two bodies is called static friction if slipping does not occur and kinetic friction if slipping does occur. The amount or magnitude of static friction between two bodies depends on the degree of the tendency to slip. Static friction is a passive force, which comes into action only to prevent a slipping that other forces tend to cause, and which is, in any given case, only so large as may be necessary to prevent that slipping. It increases as the tend­ency to slip increases, having its greatest value when slipping im­pends. Likewise, the inclination of the total reaction to the normal pressure increases as the tendency to slip increases, having its greatest value when slipping impends.

Limiting friction is a name sometimes given to the friction of impending slip. We denote it by F₀ because it is a maximum value. The coefficient of static friction for two surfaces is the ratio of the limiting friction to the corresponding normal pressure. We denote it by µ, then F₀ = µN.

The angle of friction for two surfaces is the angle between the directions of the normal pressure and the total reaction when slipping impends. We denote it by φ; then, since R may be looked on as the resultant of F₀ and N, tan φ = F₀/N and so µ = tan φ.

Laws of Friction. - The laws which determine the amount of fric­tion between the dry surfaces of solids are as follows:

1. The friction between two sliding surfaces is nearly independent of the velocity.

2. If the force perpendicular to the surface remains the same, the friction does not depend on the area of the rubbing surfaces. This statement is only approximately true.

3. The force of friction is proportional to the total force pressing one surface against another.

4. The force of friction is greater at the start than after motion has begun.

Rolling Friction,the opposition offered to the rolling of one body on another, is in general very much smaller than sliding friction. For this reason the sliding friction force exerted on a shaft rotating in bearings can be reduced by installing ball or roller bearings which replace sliding friction by rolling friction. However, the sliding fric­tion of a well lubricated shaft is already so small that there is no great gain in efficiency on mounting it in roller bearings. The chief reason for the use of such bearings in machinery is to reduce wear and to simplify lubrication problems [2, С. 64 - 66].

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