friction resistance offered to the movement of one body past another body with which it is in contact. In certain situations friction is desired. Without friction the wheels of a locomotive could not "grip" the rails nor could power be transmitted by belts. On the other hand, in the moving parts of machines a minimum of friction is desired; an excess of friction produces heat, which in turn causes expansion, the locking of the moving parts, and a consequent breakdown of the machinery. Lubrication is important in minimizing friction as are also such devices as ball and roller bearings .

Factors Affecting Friction

Friction depends partly on the smoothness of the contacting surfaces, a greater force being needed to move two surfaces past one another if they are rough than if they are smooth. However, friction decreases with smoothness only to a degree; friction actually increases between two extremely smooth surfaces because of increased attractive electrostatic forces between their atoms. Friction does not depend on the amount of surface area in contact between the moving bodies or (within certain limits) on the relative speed of the bodies. It does, however, depend on the magnitude of the forces holding the bodies together. When a body is moving over a horizontal surface, it presses down against the surface with a force equal to its weight, i.e., to the pull of gravity upon it; an increase in the weight of the body causes an increase in the amount of resistance offered to the relative motion of the surfaces in contact.

The Coefficient of Friction

The coefficient of friction is the quotient obtained by dividing the value of the force necessary to move one body over another at a constant speed by the weight of the body. For example, if a force of 20 newtons is needed to move a body weighing 100 newtons over another horizontal body at a constant speed, the coefficient of friction between these two materials is 20/100 or 0.2. Different materials in contact yield different results; e.g., different resistances are felt if one pushes a block of wood over surfaces of wood, steel, and plastic. A different coefficient of friction must be calculated for each different pair of materials.

There is more than one coefficient of friction for a given pair of materials. More force is needed to start a body moving across a surface than is needed to keep it in motion once started. Thus the coefficient of static friction (describing the former case) for a pair of substances is greater than the coefficient of kinetic friction (describing the latter case) for the substances. Similarly, sliding friction is greater than rolling friction. The force of friction between two materials can be calculated by multiplying the coefficient of friction between these materials (determined experimentally and listed in engineering handbooks) by the force holding them together (e.g., the weight of the moving body).

The Nature of Fluid Friction

Fluid friction is observed in the flow of liquids and gases. Its causes are similar to those responsible for friction between solid surfaces, for it also depends on the chemical nature of the fluid and the nature of the surface over which the fluid is flowing. The tendency of the liquid to resist flow, i.e., its degree of viscosity, is another important factor. Fluid friction is affected by increased velocities, and the modern streamline design of airplanes and automobiles is the result of engineers' efforts to minimize fluid friction while retaining speed and protecting structure.

Friction

Friction is a force that resists motion when the surface of one object slides over the surface of another. Frictional forces are always parallel to the surfaces in contact, and they oppose any motion or attempted motion. No movement will occur unless a force equal to or greater than the frictional force is applied to the body or bodies that can move.

Friction is often regarded as a nuisance because it reduces the efficiency of machines. It is also, however, an essential force for such items as nails, screws, pliers, bolts, forceps, and matches. Without friction we could not walk, play a violin, or pick up a glass of water.

Gravity and friction are the two most common forces affecting our lives. While we know a good deal about gravitational forces, we know relatively little about friction. Frictional forces are believed to arise from the forces of attraction between the molecules in two surfaces that are pushed together by pressure.

The surface of a given material may feel smooth. But at the atomic level, it is filled with valleys and hills a hundred or more atoms or molecules high. When one object is placed on top of another object, the pressure it exerts squeezes the hills and valleys in the two surfaces together. Molecules on the two surfaces tend to stick to one another, producing friction.

The laws of friction

Three laws apply to frictional forces. First, the force of friction between an object and the surface on which it rests is proportional to the weight of the object. The heavier the object, the greater the frictional force. The lighter the object, the less the frictional force. Second, in most cases, the force of friction between an object and the surface on which it rests is independent of the surface area of the object. Two objects may have very different surface areas, but if they weigh the same amount they will exert the same frictional force. Third, the force of friction between an object and the surface on which it rests is independent of the speed at which the object moves—provided that the speed is not zero.

The third law suggests that two kinds of friction exist, static and kinetic friction. Static friction is the force required to make an object at rest begin to move. In contrast, kinetic friction is the resistance to motion of an object moving across a surface. Static friction always is greater than kinetic friction because fewer areas of the object are in contact with a surface once a body is in motion. Molecules are not in contact long enough to form attractions to each other.

Two kinds of kinetic friction exist: sliding and rolling friction. When you push a book across a table top, the resistance you experience is caused by sliding friction. When you push a wheelbarrow over the ground, the resistance you feel is caused by rolling friction. In general, rolling friction is always less than sliding friction. One reason is that the contact area between a wheel and a surface is very small and brief.

The friction between two surfaces can be reduced by using a lubricant, such as oil. The oil reduces the amount of contact between two surfaces and allows them to slide or roll over each other more smoothly.

friction Resistance encountered when surfaces in contact slide or roll against each other, or when a fluid flows along a surface. Friction is directly proportional to the force pressing the surfaces together and the surface roughness. When the movement begins, it is opposed by a static friction up to a maximum ‘limiting friction’ and then slipping occurs. Aircraft reduce air (fluid) friction by having a streamlined design. Rolling friction is friction between a wheel and the surface on which it is moving.

fric·tion / ˈfrikshən/ • n. the resistance that one surface or object encounters when moving over another: a lubrication system that reduces friction. ∎  the action of one surface or object rubbing against another: the friction of braking. ∎  conflict or animosity caused by a clash of wills, temperaments, or opinions: a considerable amount of friction between father and son. DERIVATIVES: fric·tion·less adj.

friction n.
1. the resistance that one surface or object encounters when moving over another: a lubrication system that reduces friction.

2. the action of one surface or object rubbing against another: the friction of braking.

3. conflict or animosity caused by a clash of wills, temperaments, or opinions: a considerable amount of friction between commanders.
frictionless adj.

friction XVI. — F. — L. frictiō, -ōn-, f. fricāre rub; see -TION.

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