Question
List the advantages and disadvantages of friction. Give two examples of each and explain why friction is called a “necessary evil.”
Solution — Step by Step
Friction is the force that opposes the relative motion between two surfaces in contact. Before listing advantages and disadvantages, remember: friction arises because surfaces are never perfectly smooth — they have tiny bumps and irregularities at the microscopic level.
Friction is essential for most activities we take for granted:
- Walking — When we push our foot backward against the ground, friction pushes us forward. On a frictionless surface (like ice), we cannot walk at all.
- Brakes in vehicles — Brake pads press against the wheel disc; friction between them converts kinetic energy into heat and slows the vehicle down.
- Writing — The friction between a pen or pencil and paper allows ink or graphite to transfer onto the surface. Without it, the pen would glide off without leaving a mark.
- Holding objects — We can grip a glass, hold a cricket bat, or turn a doorknob only because of friction between our fingers and the object.
The same friction that helps us also causes problems:
- Wear and tear — Friction between moving machine parts wears them down over time. This is why the soles of our shoes thin out and engine parts need replacement.
- Heat generation — Friction converts kinetic energy into heat. In machines, this wastes energy and can cause overheating — a major problem in engines and motors.
- Energy loss — In any machine, a significant fraction of input energy is lost to friction. This reduces the efficiency of the machine.
- Noise — Friction between parts (like a squeaky door hinge) produces unwanted sound and vibration.
The phrase “necessary evil” means something that has harmful effects but cannot be avoided because we need it. Friction fits perfectly:
- It is necessary — we cannot walk, write, drive, or hold anything without it.
- It is evil — it wastes energy, causes wear, generates heat, and reduces machine efficiency.
We try to reduce friction where it is harmful (using lubricants, ball bearings) but increase it where it is helpful (using rough soles, brake pads, sandpaper).
Why This Works
Friction exists because of the electromagnetic forces between atoms on two surfaces. When surfaces press together, their microscopic peaks interlock. Moving them requires breaking these interlocks — that resistance is friction.
The “necessary evil” framing is NCERT’s way of capturing a real engineering trade-off. Every machine designer wants to minimise frictional losses in the drivetrain but maximise friction in the braking system — these are opposite goals for the same force.
This also explains why lubricants like oil or grease reduce friction: they fill in the microscopic gaps between surfaces, preventing the peaks from interlocking directly.
Alternative Method — Organise as a Table
For board exams, a well-drawn table earns full marks and is faster to write:
| Advantages of Friction | Disadvantages of Friction |
|---|---|
| Enables walking and running | Causes wear and tear of machine parts |
| Vehicles can brake and stop | Wastes energy as heat |
| We can write and hold objects | Reduces efficiency of machines |
| Matches, sandpaper, and grinding work | Produces noise and vibration |
In CBSE Class 8 exams, “give two examples each” questions typically carry 2 marks. Write one advantage + example and one disadvantage + example clearly in two separate paragraphs. The table format works well but only if you label it properly.
Common Mistake
Students write “friction produces heat” only under disadvantages. But friction producing heat is sometimes an advantage — matchsticks work because friction between the match head and the striking surface generates enough heat to ignite the chemicals. Always check the context before labelling something as only an advantage or only a disadvantage.