Energy & Work Done
How energy transfers and transforms in the real world
Energy is the ability to do work. It cannot be created or destroyed — only transferred from one form to another (Law of Conservation of Energy).
Types of Energy:
Work Done: When a force moves an object, work is done.
Work Done = Force × Distance (W = Fd)
Work done is measured in Joules (J).
Power: The rate of doing work.
Power = Work Done / Time (P = W/t)
Power is measured in Watts (W). 1 Watt = 1 Joule per second.
Efficiency: No machine is 100% efficient. Some energy is always wasted as heat.
Efficiency = (Useful energy output / Total energy input) × 100%
Key Points to Remember
- 1Energy cannot be created or destroyed, only transferred
- 2KE = ½mv², GPE = mgh
- 3Work Done = Force × Distance (in Joules)
- 4Power = Work Done / Time (in Watts)
- 5Efficiency = (useful output / total input) × 100%
Pakistan Example
Construction Workers at Bahria Town
A construction worker at Bahria Town lifts a 20 kg cement bag to a height of 3 metres. The work done against gravity is W = mgh = 20 × 10 × 3 = 600 Joules. If he does this in 4 seconds, his power output is 600/4 = 150 Watts. That's like powering one and a half light bulbs! Now imagine doing this hundreds of times a day — you can see why construction work is so exhausting.