Friction
Publisher Summary
This chapter discusses friction and its several types. When an object, such as a block of wood, is placed on a floor and sufficient force is applied to the block, the force being parallel to the floor, the block slides across the floor. When the force is removed, motion of the block stops. Therefore, there is a force that resists sliding. This force is called dynamic or sliding friction. A force may be applied to the block that is insufficient to move it. In this case, the force resisting motion is called the static friction or striction. There are three factors that affect the size and direction of frictional forces, namely, type of surface, size of the force acting at right angles to the surfaces in contact, called the normal force, and the direction of the frictional force is always opposite to the direction of motion
1. When an object, such as a block of wood, is placed on a floor and sufficient force is applied to the block, the force being parallel to the floor, the block slides across the floor. When the force is removed, motion of the block stops; thus there is a force which resists sliding. This force is called dynamic or sliding friction. A force may be applied to the block which is insufficient to move it. In this case, the force resisting motion is called the static friction or striction. Thus there are two categories into which a frictional force may be split:
2. There are three factors which affect the size and direction of frictional forces.
(i) The size of the frictional force depends on the type of surface (a block of wood slides more easily on a polished metal surface than on a rough concrete surface).
(ii) The size of the frictional force depends on the size of the force acting at right angles to the surfaces in contact, called the normal force. Thus, if the weight of a block of wood is doubled, the frictional force is doubled when it is sliding on the same surface.
(iii) The direction of the frictional force is always opposite to the direction of motion. Thus the frictional force opposes motion, as shown in Figure 28.1.
3. The coefficient of friction, µ, is a measure of the amount of friction existing between two surfaces. A low value of coefficient of friction indicates that the force required for sliding to occur is less than the force required when the coefficient of friction is high. The value of the coefficient of friction is given by
Transposing gives: frictional force = µ × normal force,
The direction of the forces given in this equation are as shown in Figure 28.2. The coefficient of friction is the ratio of a force to a force, and hence has no units. Typical values for the coefficient of friction when sliding is occurring, i.e. the dynamic coefficient of friction are:
4. In some applications, a low coefficient of friction is desirable, for example, in bearings, pistons moving within cylinders, on ski runs, and so on. However, for such applications as force being transmitted by belt drives and braking systems, a high value of coefficient is necessary.
Advantages and disadvantages of frictional forces
(a) Instances where frictional forces are an advantage include:
(i) Almost all fastening devices rely on frictional forces to keep them in place once secured, examples being screws, nails, nuts, clips and clamps.
(ii) Satisfactory operation of brakes and clutches rely on frictional forces being present.
(iii) In the absence of frictional forces, most accelerations along a horizontal surface are impossible. For example, a person’s shoes just slip when walking is attempted and the tyres of a car just rotate with no forward motion of the car being experienced.
(b) Disadvantages of frictional forces include:
(i) Energy is wasted in the bearings associated with shafts, axles and gears due to heat being generated.
(ii) Wear is caused by friction, for example, in shoes, brake lining materials and bearings.
(iii) Energy is wasted when motion through air occurs (it is much easier to cycle with the wind rather than against it).
6. Two examples of design implications which arise due to frictional forces and how lubrication may or may not help are:
(i) Bearings are made of an alloy called white metal, which has a relatively low melting point. When the rotating shaft rubs on the white metal bearing, heat is generated by friction, often in one spot and the white metal may melt in this area, rendering the bearing useless. Adequate lubrication (oil or grease), separates the shaft from the white metal, keeps the coefficient of friction small and prevents damage to the bearing. For very large bearings, oil is pumped under pressure into the bearings and the oil is used to remove the heat generated, often passing through oil coolers before being recirculated. Designers should ensure that the heat generared by friction can be dissipated.
(ii) Wheels driving belts, to transmit force from one place to another, are used in many workshops. The coefficient of friction between the wheel and the belt must be high, and it may be increased by dressing the belt with a tar-like substance. Since frictional force is proportional to the normal force, a slipping belt is made more efficient by tightening it, thus increasing the normal and hence the frictional force. Designers should incorporate some belt tension mechanism into the design of such a system.