The limiting frictional force between two surface depends on I. the gaseous reaction between the surfaces II…

Analyze the question

We are asked what the limiting (maximum static) frictional force between two surfaces depends on. The statements are:

• I. the gaseous reaction between the surfaces
• II. the nature of the surfaces in contact
• III. the relative velocity between the surfaces

(Option B mentions "IV", but only three statements are given, so that option appears to be a typo in the question.)

Concept

The limiting frictional force is given by:

F = μR

where μ is the coefficient of limiting friction and R is the normal reaction.

The laws of friction say:

1. Friction depends on the nature of the surfaces (rough or smooth, material type), which is captured by μ.
2. It is proportional to the normal reaction R.
3. It is independent of the area of contact.
4. It is independent of the relative velocity, for kinetic friction at moderate speeds. Limiting friction is the maximum friction just before motion starts, when the relative velocity is zero anyway.

Checking each statement

• I. Gaseous reaction between the surfaces: this has no role in the laws of friction, so it is false.
• II. Nature of the surfaces in contact: this determines μ, so it is true.
• III. Relative velocity: friction does not depend on it, and limiting friction applies at the point of impending motion, so it is false.

Only statement II is correct.

Answer: C (II only)

Exam tip: Remember that friction depends on nature of surfaces and normal reaction, but NOT on area of contact or speed. If you see "area" or "velocity" in a friction question, it is usually a distractor.