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What happens to the true area of contact when the normal force is increased?
AIt falls as the surfaces harden
BIt stays fixed whatever the load
CIt increases as high spots flatten
DIt becomes equal to the total area
Answer & Solution
Correct answer: C. It increases as high spots flatten
1. Only the high spots of two rough surfaces are genuinely touching.
2. A greater normal force presses them together harder and brings more of them into contact.
3. So the actual contact area increases with the normal force.
4. Since friction follows that real area, friction rises with the normal force too.
_Source: OpenStax College Physics (CC BY 4.0), Ch 5 "Further Applications of Newton's Laws: Friction, Drag, and Elasticity", section Making Connections: Submicroscopic Explanations of Friction_
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