Distinguish between static and kinetic friction. Why is rolling easier than sliding? Give two ways to reduce friction.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define friction. Friction is a contact force along the surfaces in contact that opposes their relative motion.
2 · Static friction. It acts on a body at rest and adjusts to the applied force, up to the limit fₛ(max) = μₛFₙ.
3 · Kinetic friction. It acts on a sliding body: fₖ = μₖFₙ. Since μₖ < μₛ, it is smaller than limiting friction.
4 · Draw the graph. [Friction on the y-axis, applied force on the x-axis: a rising straight line marked 'at rest' up to fₛ(max), then a small drop to a flat line fₖ marked 'sliding']
5 · Rolling. For the same normal force, rolling friction is far smaller than sliding friction, so wheels make loads easy to move.
6 · Ways to reduce friction. Ball bearings in fans and bicycles; oil or grease in engines.
7 · Close in one line. So friction can be reduced, but never removed completely.
Scoring tip: Write both formulas, μₛFₙ and μₖFₙ, and state μₖ < μₛ in words. The graph earns a mark even if your text is short.
