Brakes of very small contact area are not used although friction is independent of area, because friction

The correct answer is: Causes wear and tear.

This question tests a subtle but important concept in the practical application of friction. Let's break it down step by step.

Step 1: What does "friction is independent of area" mean?

According to the classical laws of friction (Amontons' Laws), the magnitude of the friction force $F_f$ depends only on the normal force $F_N$ and the coefficient of friction $\mu$, not on the contact area $A$.

F_f=\mu \times F_N

So, theoretically, whether the brake pad has a large or a very small contact area, the braking (friction) force should remain the same for a given normal force.

Step 2: If friction is the same, why not use very small contact areas?

The key lies in understanding pressure. When the contact area is very small, the same normal force is distributed over a much smaller area, which means the pressure at the contact surface increases dramatically.

\mathrm{Pressure}=\frac{\mathrm{Force}}{\mathrm{Area}}

So if the area $A$ is very small, the pressure $P$ becomes very large even for a moderate normal force.

Step 3: How does high pressure lead to wear and tear?

This extremely high pressure at the contact interface causes the surface molecules of both the brake material and the surface it acts on (e.g., a drum or disc) to be under intense stress. This results in:

• Rapid abrasion — the material at the tiny contact point gets scraped and eroded very quickly.
• Excessive heat generation — the high pressure concentrates frictional heat in a very small region, accelerating material degradation.
• Rapid deterioration of the brake pad and the braking surface.

In other words, even though the friction force is the same, the pressure is inversely proportional to the area. A very small area means very high pressure, causing the materials to wear out extremely fast, making the brakes unreliable and short-lived.

Step 4: Why the other options are incorrect?

• "Resists motion" — This is the general definition/purpose of friction, not a reason to avoid small contact areas.
• "Depends upon the nature of materials" — This is a true property of friction (it affects the coefficient μ), but it is not the reason small-area brakes are avoided.
• "Operating in this case is sliding friction" — Brakes always use sliding friction regardless of the contact area, so this is not a distinguishing reason.

Conclusion: Brakes with very small contact areas are not used because, even though the frictional force remains the same, the extremely high pressure at the tiny contact surface causes rapid and excessive wear and tear, drastically reducing the life and effectiveness of the brake system.