Page 168 - WCS - Electrical

WORKSHOP CALCULATION & SCIENCE - CITS

EXERCISE 15 : Friction

Heat & Temperature
Introduction
It is a fact that all surfaces of the bodies are never perfectly smooth. It has been observed that whenever, even a
very smooth surface is viewed under a microscope, it is found to have some roughness and irregularities, which
may not be detected by an ordinary touch if a block of one substance is placed over the level surface of the
same or of the different material, a certain degree of interlocking of the minutely projecting particles takes place.
This does not involve any force so long as the block does not move or tends to move But whenever one of the
blocks moves or tends to move tangentially with respect to the surface, on which it rests, the interlocking property
of the projecting particles opposes the motion. This opposing force, which acts in the opposite direction of the
movement of the block, is called “Force of friction or simply ‘friction’
The friction may be classified into two categories
(i) Static friction and (ii) Dynamic friction The friction experienced by a body when it is at rest or when the body
tends to move is known as static friction. The friction experienced by a body when it is in motion, is known as
kinetic friction
Advantages of Friction
1 Floors are made rough to avoid slipping
2 Belts and ropes moving on pulley
3 Frictions clutches.
4 A ladder leaning against a wall
5 Brakes are applied to the wheels of vehicles to stop them.
6 Vehicles move due to friction between revolving wheels with tyres and the road.
7 Heat is generated when two rough surfaces are rubbed against each other
Disadvantages of Friction
1 Wear and tear of all sliding surfaces in contact.
2 Reduction in efficiency of the machines.
3 Generation of heat requiring use of coolants in Machining operations
Methods of Reducing Friction
1 Use of lubricants.
2 Use of ball and roller bearings
3 Polishing
Normal Reaction
When a solid substance is placed on an even surface solid exerts pressure on the surface equal to its or weigh
The surface also reacts on the solid The amours u downward force exerted by the solid on the surface, an
equal amount of force acts upward venically from the surface. This reaction is called normal reaction. This is
represented by ‘ as is shown in Fig. below. Therefore. R=Wand F=p.
Limiting Friction
When two dry surfaces move striking each other, then friction is created between them. As the force is increased,
the friction also increases
A stage in this increase reaches to a point, where the friction becomes maximum tending to stop the movement of
the solid. This maximum friction is called limiting friction. The limiting friction is equal to the outside force applied
when the object is just about to move. The limiting friction force is represented by Fas is shown in Fig above

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