Candela (cd) is SI base unit for luminous intensity. Lumen: luminous flux; Lux: illuminance; Watt: power. Memory aid: 'Candela = candle power; base unit for perceived light'. Unit knowledge frequently tested in competitive exams.
Explanation:
Work W = F·d = Fd cosθ. cos0° = 1 gives maximum work. At 90°, cos90° = 0, work zero. For given magnitudes, maximum when force parallel to displacement. This is basic definition.
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