Question

Frictional force of granular material is -

(A) Proportional to normal load

(B) Independent of the area of the sliding surface

(C) Proportional to actual area of contact

(D) Dependent on nature of material in contact

Choose the correct answer from the options given below:

• A. (A) and (C) only
• B. (A), (B), (C) and (D)
• C. (A), (B) and (D) only
• D. (C) and (D) only

Hint:

Recall the three classical laws of friction taught for sliding solids and check each statement against them.

Answer 1

The Correct Option is C

Write the friction force using the standard Coulomb friction model.

\[F = \mu N\]

This immediately shows F depends on the normal load N, so (A) holds. It also shows F does not contain a term for the nominal contact area, so friction does not change if you just spread the same load over a larger apparent surface, confirming (B).

The coefficient \(\mu\) itself is a property of the pair of materials sliding against each other, rougher or stickier materials give a higher \(\mu\), which confirms (D).

Statement (C) claims F scales with the actual area of contact, but the model above has no such term, it directly contradicts the independence stated in (B), so (C) is false.

\[\boxed{\text{Option 3: (A), (B) and (D) only}}\]