JEE Main 2024 · 27 January · evening · Single correct · +4 / −1
Given below are two statements :
Statement (I) : The limiting force of static friction depends on the area of contact and independent of materials.
Statement (II) : The limiting force of kinetic friction is independent of the area of contact and depends on materials.
In the light of the above statements, choose the most appropriate answer from the options given below :
- (a) Statement I is incorrect but Statement II is correct
- (b) Both Statement I and Statement II are correct
- (c) Both Statement I and Statement II are incorrect
- (d) Statement I is correct but Statement II is incorrect
Answer and solution
Answer (a) Statement I is incorrect but Statement II is correct
Solution
Physics sir · 3 steps
Friction cares about how hard the surfaces press and what they are made of, not how big the contact patch is.
- Laws of friction. Limiting static friction is fₛ = μₛ N and kinetic friction is fₖ = μₖ N. Neither formula has an area term in it.
- Where materials enter. The coefficients μₛ and μₖ depend on the two surfaces in contact: their materials and how rough they are. So both kinds of friction depend on materials.
Statement Area of contact Materials Verdict I (static) claims it depends, but it does not claims it does not depend, but it does incorrect II (kinetic) independent, as claimed depends, as claimed correct - So. Statement I gets both halves wrong and Statement II gets both halves right, so the answer is (a).
The idea to keep. For both static and kinetic friction, think f = μ N: the normal force and the pair of materials (through μ) decide it. The area of contact does not appear anywhere.
Answer from the NTA final answer key. Solution by PrepWiser.