The correct arrangement is as follows: Wet marble floor, dry marble floor, newspaper, towel.

We know that friction is caused by the interlocking of the irregularities in the two surfaces. It is obvious that the force of friction will increase if the two surfaces are pressed harder. So it is easy to drag a mat when nobody is sitting on but difficult when someone is sitting on it but difficult when someone is sitting on it.

The boy having the tyres which are old and used is most likely to slip because these tyres will experience less frictional force which is insufficient to move on the oily road.

Yes, there is always a force of friction between the wheels of a moving train and iron rails. The name of this friction is rolling friction, since, the wheels are rolling on the track. On introducing air cushion, the frictional force becomes less, since there is no contact between rails and wheels.

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.

(i) Rolling: The rolling wheels attached to luggage bags make it comfortable for passengers to carry them because rolling reduces friction and is smaller than sliding friction.
(ii) Sliding: By polishing surfaces or by applying a lubricant.

The substances which reduce friction are called lubricants e.g., oil creams and grease.

Liquids and gases can flow and they do not have definite shape. So, they are called as fluid.

When an object moves in a fluid, then the resistance to the movement of object exerted by fluid is called fluid friction.

Frictional force helps to write by chalk on blackboard.

Friction may be reduced in the following ways:
(i) Polishing the surface: We polish the rubbing surfaces to reduce their unevenness and make them smooth.
(ii) Lubricating the surface: To reduce friction in order to increase efficiency when oil, grease or graphite is applied between the moving parts of a machine, a thin layer is formed there and moving surfaces do not directly rub against each other. These substances are lubricants which reduce the friction.
(iii) Using wheels and ball bearings: By using wheels and ball bearing we convert sliding friction into rolling friction. This reduces friction between the two contact surfaces and helps us to save energy, effort and time.
(iv) Streamlining: Objects are given special shapes, such as streamlined body to aeroplanes. Their bodies shapes make them lose less energy in overcoming friction. Hence, all vehicles are designed to have shapes that reduce fluid 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.