Home › UAE MoE Grade 12 › Physics › Friction, Drag and Elasticity › When two rough surfaces touch, how much of the t…
When two rough surfaces touch, how much of the total area is really in contact?
AAlmost all of the total area
BA tiny fraction of the total
CExactly one half of the area
DNone of it, they float on air
Answer & Solution
Correct answer: B. A tiny fraction of the total
1. A surface that looks flat is covered in microscopic high and low spots.
2. Only the high spots actually meet, so the true contact area is a tiny fraction of the whole.
3. Pressing harder flattens more high spots together and raises that true contact area.
4. Friction turns out to be proportional to this real area, which explains the normal force rule.
_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_
Related questions
The tip of a probe dragged across a material is deformed by atomic-scale friction. The forWhat happens to the downward force of gravity on a skydiver as their speed rises?Which two forces act on a skydiver falling through air, once buoyancy is ignored?What would a nearly friction-free environment save on a large scale?What kind of characteristics of friction are the simple rules taught at school?Which of these is the standard everyday example of kinetic friction acting?Why do two surfaces become warm when they are rubbed together?Adding a heavy box on top of a crate makes it harder to push. Why?