Q1.
Does this mean that the force of friction will be greater if the surfaces are rough?
Answer
Yes. Friction is greater on rough surfaces, and Activity 5.4 shows it: on sand the box stops almost at once, on glass it slides far.
Why it happens: friction arises from the irregularities of the two surfaces in contact. Even surfaces that appear smooth have a large number of minute bumps and hollows (Fig. 5.6). When two surfaces are placed in contact, these irregularities lock into each other and oppose any effort to move one surface over the other. A rough surface has bigger and deeper irregularities, so more of them interlock and more firmly — the opposition to sliding, that is the friction, is therefore greater.
Did you know? This is why the sole of a new sports shoe is deeply patterned and why a well-worn one slips. It is also why we sprinkle sand on a wet slope and why carrom players smooth the board with boric powder — to increase friction where we want a grip, and reduce it where we do not.