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According to the atomic-scale discussion of friction, how does the visible contact area between two surfaces affect the friction force between them?
AFriction is directly proportional to the visible contact area
BFriction does not depend on the visible contact area at all
CFriction only changes when the visible area is doubled exactly
DFriction depends on visible area but never on the normal force
Answer & Solution
Correct answer: B. Friction does not depend on the visible contact area at all
1. The source calls out this fact as counterintuitive: friction is proportional to the normal force but not to the visible area in contact.
2. Only a tiny fraction of the visible surface actually touches, since real surfaces are rough at a microscopic level.
3. So doubling the visible contact area, without changing the normal force, does not change friction.
4. Friction does depend on the normal force, so the option that denies any dependence on the normal force is wrong.
_Source: OpenStax College Physics (CC BY 4.0), Ch 5 "Further Applications of Newton's Laws: Friction, Drag, and Elasticity", section 5.1 Friction_
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