What does power measure?

The total amount of energy stored in a system How quickly work is done — the rate of energy transfer The force an engine can exert The strength of an object — how much weight it can support

What is the SI unit of power, and who is it named after?

The joule (J), named after James Prescott Joule The newton (N), named after Isaac Newton The watt (W), named after James Watt, the Scottish engineer who improved the steam engine The pascal (Pa), named after Blaise Pascal

One watt is equivalent to:

One newton of force applied over one metre One kilogram moving at one metre per second One joule of energy transferred per second One joule of energy stored per kilogram

A machine does 900 joules of work in 30 seconds. What is its power output?

3 W 30 W 270 W 27,000 W

Two cranes each lift an identical 1000 kg load to the same height. Crane A takes 10 seconds; Crane B takes 30 seconds. How do their power outputs compare?

They have the same power — they do the same work Crane B has more power — it takes longer, so it works harder Crane A has three times the power of Crane B — it does the same work in one-third the time Crane A has nine times the power — time is squared in the power formula

A 100 W incandescent light bulb and a 10 W LED produce approximately the same brightness. What does this tell us about the LED?

The LED produces less light than the bulb — 10 W cannot match 100 W The LED is roughly ten times more efficient — it converts a much higher fraction of electrical energy into light rather than wasting it as heat The bulb and LED are equally efficient — they produce the same brightness The LED has higher voltage, which makes low power appear brighter

A city car and a sports car have the same legal top speed on public roads. Why does the sports car's higher engine power still matter in practice?

The sports car has a heavier engine, which gives it more momentum Higher power allows much faster acceleration to reach that top speed — more energy is transferred to the car per second It does not matter — once the speed limit is set, power is irrelevant The sports car uses more fuel, which releases more energy into the air

Why did James Watt use 'horsepower' as a unit when marketing his steam engines?

Horses were the fastest animals, so horsepower was the maximum possible unit Horses were the standard measure of work capacity at the time; expressing his engine's output in horsepower let potential buyers immediately understand how many horses the machine could replace The Royal Society required all power units to be named after animals Watt measured the power of his engine and it happened to equal exactly the power of one horse

A 20 W phone charger runs for 300 seconds. How much energy does it transfer to the phone?

15 joules 320 joules 6,000 joules 60,000 joules

An athlete can sprint at over 1,000 W for a few seconds but can only sustain around 200 W for a long-distance race. Why can they not maintain the sprint power level?

Athletes get tired and lose the will to keep sprinting The muscles overheat at high power and must be cooled down Sprinting draws on ATP stored in muscles and fast anaerobic pathways — these provide energy very quickly but are exhausted in seconds; sustained aerobic respiration produces energy more slowly but continuously The heart cannot pump blood fast enough for more than a few seconds