Question

Which of the following cannot be used as a lubricant?
A. Grease
B. Powder
C. Air
D. Paper

Answer 1

Hint: Lubricants are substances applied on huge machines in factories to reduce the friction between two objects which are in direct contact with each other. Lubricants are usually liquid or semi solid in nature, but they can also be found in solid and gaseous form as well in limited cases.

Complete answer:
All the substances are rough, even the smoothest of the surfaces are rough when seen at a microscopic or an atomic level. When two rough surfaces slide against each other the asperities of the surfaces come close to each other and interlock, adhere and generate friction. The goal of any lubrication approach is to separate the rubbing surfaces by a lubricant layer, which prevents or minimizes direct contact of the two bodies. Hence, lubricants can be defined as substances used to control, and mostly to reduce friction or wear and tear of the surfaces in a contact of the bodies in relative motion. Lubricants can be solid, liquid or gaseous in nature.
Grease oil is used as lubricant on wheels of the machines for its proper functionality and reduces friction between the parts of machines. Grease oil is a liquid based lubricant.
Powder is a solid based lubricant which is often spread on a rough surface, for example a carrom board to decrease the friction between the pieces and the carrom board.
Air is a gas based lubricant which protects the surfaces from the attack of chemically active substances, such as oxygen, by creating a corrosion resistant layer.
Paper is not a lubricant as it is itself a rough surface, and cannot be used as a lubricant.

So, the correct answer is “Option D”.

Note:
Lubricants are very much necessary in industries because huge machines create a lot of friction and would get damaged easily. Also we can get confused if air is a lubricant or not, but it is a gaseous lubricant and is used in many industries. In fact, when a person rubs his/her hands, the friction between the hands is reduced due to air itself.