01 Key Concepts
Power Definition
The rate at which work is done or energy is transferred: P = W/t, where W is work and t is time.
Units of Power
Measured in watts (W), where 1 watt = 1 joule per second.
Power and Velocity
For a constant force, power can also be written as P = F*v, where v is the object's velocity in the direction of the force.
Average vs. Instantaneous Power
Average power is total work divided by total time; instantaneous power is the power at one specific moment, which may vary if force or velocity changes.
Efficiency
The ratio of useful power output to total power input, often expressed as a percentage; no real machine is 100% efficient due to losses like friction and heat.
02 Key Formulas
- P = W/t
- P = F*v
03 Solved Examples
Example 1 A motor does 500 J of work in 10 seconds. Find its power output.
- Apply P = W/t = 500/10.
Answer: 50 W
Example 2 A car engine exerts 2000 N of force while moving at 15 m/s. Find the power output.
- Apply P = F*v = 2000*15.
Answer: 30,000 W (or 30 kW)
Example 3 A machine has power input 1000 W and useful power output 750 W. Find its efficiency.
- Efficiency = (useful output/input)*100% = (750/1000)*100%.
Answer: 75%
04 Practice Questions
1A machine does 1000 J of work in 20s. Find power.
50 W
2A force of 500 N moves an object at 4 m/s. Find power.
2000 W
3A machine has input 500 W and output 400 W. Find efficiency.
80%
4What is the SI unit of power?
The watt (W)
5What is the relationship between power, work, and time?
P = W/t
📄 Power — Downloadable Worksheet
10 questions with a full answer key. Grab the PDF to print, or try the interactive version in your browser.