Example 1 · level 1
A water pump does 1,040 J of work in 4 s. Find its power (in W).
Show solution
- P = 1,040 ÷ 4 = 260 W.
Answer: 260 W
CLASS 9 SCIENCE · WORK AND ENERGY
Power and the kilowatt hour
Power = work ÷ time (watts). 1 kWh = energy used by a 1000 W appliance in 1 hour = 3.6 × 10⁶ J; this is the "unit" on electricity bills.
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Worked examples
Example 2 · level 2
Arjun (mass 25 kg) runs up a staircase of height 12 m in 5 s. Find the power developed (in W, correct to 2 decimal places). Take g = 10 m/s².
Show solution
- W = 3000 J.
- P = 3000 ÷ 5 = 600 W.
Answer: 600 W
Example 3 · level 3
A school lab uses 5 appliances of 40 W each for 8 hours a day. How many units (kWh) are used in 1 day?
Show solution
- Total power = 0.2 kW.
- Time = 8 × 1 = 8 h.
- Energy = 0.2 × 8 = 1.6 kWh.
Answer: 1.6 kWh