Question

(a) Explain how lubrication reduces friction?
(b) Explain with one example that frictional force is proportional to the normal force.

Answer 1

Hint:Friction is caused by the irregularities present on the surfaces and if one has to reduce the friction then he should reduce those irregularities’ effects which is effectively done by the lubricants. Friction is caused by the irregularities on the two surfaces in contact.

Complete answer:
Friction: Friction is an intermolecular force between two bodies that opposes the relative motion or tendency of relative motion between two surfaces in contact. The frictional force is usually larger on the rough and smaller on the smooth surfaces.

Disadvantages of friction:
-A significant amount of energy is wasted in the form of heat to overcome the force of friction.
-Friction causes a lot of wear and tear in the moving parts of the machine due to which its maintenance is required.
-The efficiency of the machine decreases due to the presence of force of friction.
-To overcome these disadvantages discussed above lubricants are used.

Lubricant: A lubricant is a substance which is used to reduce friction between two or more surfaces in mutual contact. As a result, the heat generated is lowered when the surfaces move. It is one of the most important substances used in machines to improve the overall performance of machines and increase their working life like automobiles and other heavy machineries. Different varieties of lubricants such as Engine Oil, Hydraulic Oil and Greases are used in various automobiles, industries etc.

Types of lubricants:
-Oil
-Grease
-Dry lubricants – like graphite

Working of Lubricants: When we put lubricants to a machine’s part, it gets spread between the surfaces which are in contact with each other and removes the irregularities present on the surface by forming a thin layer between the surfaces in contact. As a result, the contact between the two hard irregular surfaces is replaced by a lubricant layer hence the frictional force is reduced significantly.

(b) As normal force increases the extent of interlocking of surfaces between contact is increased which makes frictional force vary directly proportional to normal force between the two surfaces in contact. Mathematically, $$f=\mu N$$ , as $$\mu$$ is fixed for a surface so $$f\propto N$$.
Example: pulling a block of $$2kg$$on a rough surface is tougher as compared to pulling a block of $$1kg$$ this happens because as the mass of block increases its weight also increases resulting in increase of normal contact force and eventually the force of friction between the block and surface.

Note: As we know the fact that friction a necessary evil so every time, we are not interested in reducing the friction sometimes we need to increase the friction too like;
-The tyres of the automobiles are made rough or are treated to increase the force of friction for better grip of the tyres on slippery roads due to water or sand.
-For better braking in automobiles brake shoes are also made rough.