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Friction: the Force That Both Stops and Starts Motion
Friction is the contact force that opposes sliding between two surfaces. Without it you could not walk, a car could not brake, a nail would slip out of wood and a knot would fall apart. With too much of it, machines heat up and wear out. Exams therefore ask about both faces of friction: the useful one and the wasteful one.
The whole chapter rests on one formula, f = μN, where μ is the coefficient of friction and N is the normal reaction from the surface. Notice what is missing: contact area and speed do not appear. That single observation answers a surprising number of true-or-false questions.
In this short we separate static, kinetic and rolling friction, solve the incline and braking-distance problems with numbers you can check, and finish with the traps that competitive papers keep repeating.
1. A 10 kg block rests on a floor with μs = 0.5 (g = 10 m/s²). It is pushed horizontally with 30 N and stays at rest. What is the friction force on it?
2. Which of the following frictions has the smallest value for the same pair of surfaces and load?
3. A block just starts to slide down a rough incline when the slope is 30°. What is the coefficient of friction between the block and the incline (take tan 30° = 0.577)?
4. A car moving at 20 m/s brakes with all wheels locked on a road where μ = 0.5. Taking g = 10 m/s², the stopping distance is:
5. A rectangular wooden block is pulled at steady speed first on its large face and then on its small face over the same table. The friction force in the two cases is:
6. A 20 kg block rests on a 60° incline (g = 10 m/s²). The normal reaction exerted by the incline on the block is:
7. Ball bearings are used in the axles of machines mainly because they:
8. A 5 kg block on a floor with μk = 0.2 is pulled by a constant horizontal force of 20 N (g = 10 m/s²). Its acceleration is: