Work and power
In physics work is force times the distance covered, and it is measured in joules, the same unit as energy, because doing work is exactly transferring energy. One consequence jars: holding a suitcase still for a minute is no work at all, however tiring. Power is that work divided by the time it takes, which is why two people climbing the same stairs do the same work even though the one who runs develops more power and ends up out of breath.
Where this sits
- Comes after
- Energy
Exercises from this rung
Generated by the app, from easiest to hardest
You hold a suitcase still for a minute. How much work do you do on it?
- None at all
- It depends how heavy it is
- Its weight times a minute
- Its weight times its height
Why None at all, however tired you get: with no displacement there is no work. What tires is your muscle, spending energy to hold tension, but it does nothing to the suitcase. A table holds the same suitcase all night and spends nothing.
easierYou spend 3000 J in 5 s. What power is that?
Answer 600
Why Power is energy over time: 3000 ÷ 5 = 600 W. Two people climbing the same stairs do the same work; the one who runs up develops more power, which is why they end up out of breath.
in betweenA machine takes in 100 J and uses 20 J. What is its efficiency?
Answer 20
Why 20 out of 100 is 20%. The rest does not vanish — energy is never destroyed — it ends up as heat, which is where all energy always ends up. A petrol engine runs around 30%; an old light bulb, around 5%.
harder