Since, due to friction, a pencil writes on paper but there is smoothness on the surface of glass, a pencil cannot write on glass.

Marble will move the shortest distance on the layer of sand because it will exert a greater force of friction on the marble and other two surfaces like silk cloth and glass sheet will exert a lesser friction force comparatively.

The five smooth surfaces are
(i) Glazed tiles
(ii) Surface of an oily plot
(iii) Surfaces of wet soaps
(iv) Mirror surface
(v) Floor of polished room

We know that friction is caused by the interlocking of irregularities in the two surfaces. In case of static friction, interlocking between two objects is very strong but in case of sliding friction, there is a less time of getting interlocked with each other. Thus, the sliding friction is less than the static friction.

The object would never stop if there was no friction.

Oil, creams and grease when applied on a surface make it smooth, thus reduce friction, so these are called lubricants.

The rolling friction comes into play when one body rolls over the surface of another.

Frictional force depends upon smoothness/roughness of surfaces and their mass.

Firstly, hold a pencil or pen. Now, put it on a table. Slightly push the pencil and see how much distance it covers. Put the marking of the point where the pencil stops after covering the distance. Now spread polythene on the same table. Now, again hold the pencil and put it at the same initial point. Now, push the pencil slightly and let it stop on its own as done earlier. Now, mark the point where the pencil stops. Notice the difference in the distance. In second chance, the pencil has covered more distance. This means it had to face less friction in second attempt. It shows that smoothness of the surface reduces friction.

When a fine powder sprinkled on the carrom board and a few drops of oil are poured on the hinges of a door, the door moves smoothly. A cycJe or a motor mechanic some times uses grease between the moving parts of the cycle. In all the above cases, our try was to reduce friction in order to increase efficiency.
Also, interlocking of irregularities is avoided to a great extent due to which the movement becomes smooth. The substances which reduce friction are called lubricants. In some machines, it may not be advisable to use oil as lubricant. There is an air cushion between the moving parts, that is used to reduce friction.

As friction opposes the relative motion between the two surfaces also produce heat. So, when we rub our hands together vigorously for few minutes, we feel hotness when we strike a matchstick against the rough surface due to which it catches fire. In machineries also, heat is produced due to friction as the machine parts rub against each other.