Work, energy and power with worked conservation problems
Energy methods shortcut long force problems — if you can spot when energy is conserved and when friction removes it.
What this covers
- Work done by a force
- Kinetic and gravitational potential energy
- Conservation of energy
- Power and efficiency
- Energy lost to friction
Worked example
Problem
A 2 kg ball is dropped from 5 m. Find its speed just before impact (g = 9.8).
- 1Choose the methodNo friction, so mechanical energy is conserved.
- 2Equate energiesmgh = ½mv², and mass cancels.
- 3Solvev = √(2gh) = √(2 × 9.8 × 5) = √98.
Answer: ≈ 9.9 m/s
Mistakes that cost marks
- Using conservation of energy when friction is present.
- Measuring height from the wrong reference level.
- Confusing power (watts) with energy (joules).
Questions students ask
When should I use energy instead of forces?
When the question gives speeds and heights but not time — energy skips the intermediate motion entirely.
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