Lesson: Chapter 5 — Friction

5.3. What limiting friction depends on 4 of 4

What matters, what does not

Three activities, three answers. Two of the factors matter and one does not — and the one that does not is the one worth memorising.

Factor	Affects limiting friction?	Shown by
Nature (roughness) of the surfaces	Yes — rougher gives more friction	Activity 02
Area of the contact surfaces	No — no effect at all	Activity 03
Normal reaction between the surfaces	Yes — larger R gives more friction	Activity 04

The section in one sentence

Limiting friction is affected by the nature of the contact surfaces and by the normal reaction between them, and is not affected by the area of the contact surfaces.

Two everyday consequences

Wide tyres are not about friction

A racing car's wide tyres do not grip because more rubber touches the road — activity 03 rules that out. Their real advantages are wear and heat, spread over more rubber.

A loaded lorry is harder to shift

Load a vehicle and the normal reaction at every wheel rises, so friction rises with it. That is activity 04 at full size, and it is also why a loaded lorry needs a longer distance to stop.