Notes
Power is the rate of doing work.
- $\displaystyle P = \frac{W}{t}$. Units: watts (W); 1 W = 1 J/s.
- Example: Doing 600 J of work in 12 s gives $\displaystyle P = \frac{600}{12} = 50$ W.
- Rearranged: $W = Pt$. A 60 W bulb uses 60 J every second.
- 1 kilowatt (kW) = 1,000 W. Horsepower is another unit (about 746 W).
Practice
A machine does 500 J of work in 10 s. What is its power?
$\displaystyle P = \frac{500}{10} = 50$ W.
How much work does a 100 W motor do in 30 s?
$W = Pt = 100 \times 30 = 3000$ J.
Two students each climb the same stairs. Student A takes 10 s and Student B takes 15 s. Which statement is true?
The work is the same, but A does it in less time.
Ready for more? Take the Unit 8 test.