Which of the following statements about static friction is correct? It

Step 1: Analyze the question We need to pick the true statement about static friction, which is the friction acting between two surfaces that are not sliding relative to each other.

Step 2: Concept The limiting (maximum) static friction is given by:

F = μR

• μ = coefficient of friction, which depends on the nature of the two surfaces (rough, smooth, material)
• R = normal reaction, which on a horizontal surface equals the weight of the body (R = mg)

Friction is roughly independent of the area of contact.

Step 3: Check each option

• A. Independent of the nature of the surfaces. False. μ depends on the materials and roughness of the surfaces.
• B. Depends on the area of contact. False. For the same weight, the friction force is essentially the same whether the body rests on a large face or a small face.
• C. Depends on relative motion. False. Static friction applies when there is no relative motion. It is kinetic friction that applies during motion, and it is generally lower than static friction.
• D. Depends on the weight of the body. True. Since F = μR and R = mg on a level surface, a heavier body gives a larger normal reaction and therefore a larger friction force.

Step 4: Answer The wording "moving body" is a little loose, but D is the only statement that is true, because friction is proportional to the normal reaction, which comes from the weight.

Exam tip: Remember "N-W-A": friction depends on the Nature of the surfaces and the normal reaction (Weight), but not on Area or speed. Eliminate any option that says "area" in friction questions.