Friction and bearings — Engineering, 11–13 years
Engineers cannot remove friction everywhere, but they can control it. Bearings help moving parts slide or turn with less wasted force.
What friction does
Friction is the rubbing that resists motion when two surfaces touch. It can help, as with shoe soles gripping the ground, or waste energy, as with a bicycle chain getting warm. Engineers choose surfaces, lubricants or bearings to get the amount they need.
Why control it?
A machine with too much friction is hard to start, slows down and heats up. People first noticed this when moving heavy objects by dragging them; rollers made the job easier. A bearing solves the same problem inside a machine by replacing much sliding with easier rolling.
Comparing a bearing
A toy wheel needs a 6-newton push to turn on a rough axle. With a small bearing, it needs only 2 newtons. Step 1: measure both pushes with the same wheel and load. Step 2: subtract 2 from 6, so 4 newtons are saved. Step 3: remember that the bearing reduces resistance; it does not create energy.
The sensible mistake
It is tempting to think that all friction is bad, so every surface should be made as smooth as possible. That would make walking, braking and gripping difficult. The useful question is not “How do we remove friction?” but “Where should friction be high, and where should it be low?”
Where it appears
Bearings are inside bicycle wheels, skateboards, fans, washing machines and electric motors. Lubricant in a car engine lets metal parts move without wearing away as quickly. Shoe soles and tyre treads do the opposite: their patterns are designed to keep useful friction with the ground.
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