Ball bearings are used in the wheel axles of bicycles and in motors.  Their main purpose is to:
AMake the wheel spin faster than it normally would
BMagnetise the wheel
CAdd weight to the wheel for better balance
DReplace sliding friction (between axle and frame) with rolling friction, which is much smaller
Answer & Solution
Correct answer: D. Replace sliding friction (between axle and frame) with rolling friction, which is much smaller
Without ball bearings, the axle would simply slide against the inner surface of the hub — a lot of sliding friction and heat. The small steel balls between axle and hub roll, so the contact friction is rolling friction, which is dramatically smaller. This makes machines spin freely and last longer.
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