NECO 2001 Physics · Question 7

Which is NOT one of the laws of solid friction?

Options

A.The frictional force between two surfaces opposes their relative motion
B.The ratio of frictional force to the normal reaction varies for two given surfaces
C.The ratio of frictional force to the normal reaction is constant for two given surfaces
D.The limiting frictional force is independent of the area of surface in contact.
E.Dynamic frictional force is independent of the relative velocity of the surfaces in contact.

Answer: B. The ratio of frictional force to the normal reaction varies for two given surfaces

Explanation written with AI and checked against the answer key

Analyze the question: We need the statement that is NOT a law of solid friction. Four options are true laws and one is false.

Concept: Laws of solid friction

Friction opposes the relative motion (or tendency of motion) between two surfaces in contact.
The limiting frictional force F is directly proportional to the normal reaction R. So F/R = μ, a constant for two given surfaces.
The limiting frictional force is independent of the area of contact (provided R stays the same).
Dynamic (kinetic) friction is independent of the relative velocity of the surfaces (approximately, at moderate speeds).

Check each option:

A. Friction opposes relative motion. This is a law. ✔
B. F/R varies for two given surfaces. This contradicts the law that F/R = μ is constant. ✘ This is NOT a law.
C. F/R is constant for two given surfaces. This is the law, with μ as the coefficient of friction. ✔
D. The limiting friction is independent of contact area. This is a law. ✔
E. Dynamic friction is independent of relative velocity. This is a law. ✔

Answer: Option B is the statement that is not a law of solid friction.

Tip: In a "which is NOT" question, look for pairs of opposite statements. Options B and C contradict each other, so one of them must be the false one. Remember that μ = F/R is constant for a given pair of surfaces, so the statement saying it varies is the odd one out.

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