Knowledge Organiser: Work Done and Energy Transfer
Key Terms
- Work done: Energy transferred by a force (J)
- Power: Rate of doing work (W = J/s)
- Joule: 1 J = 1 N × 1 m
- Elastic PE: Energy stored in stretched/compressed material
Key Facts
- Work done = energy transferred
- No movement = no work done
- Force perpendicular to motion = no work done
- Work done against friction → thermal energy
Key Equations
- W = Fs (work done)
- P = W/t (power)
- Ep = ½ke² (elastic PE — higher)
- 1 W = 1 J/s; 1 J = 1 Nm
Exam Tips
- Distance must be in direction of force
- Use weight (N) not mass (kg) for lifting
- Convert time to seconds for power
- Work = energy transfer (mention this in 3+ mark answers)
Common Mistakes
- Using the wrong distance: The distance in W = F × d must be in the direction of the force — if a force is horizontal, use horizontal distance, not total path length
- Using mass instead of weight for lifting: When calculating work done lifting an object, use weight (N) = mass × g — not mass in kg directly
- Saying no work is done when carrying an object horizontally: If you carry an object at constant height with no horizontal motion, no work is done against gravity — work is only done by the horizontal force over horizontal distance
- Confusing work done and power: Work done (J) is the total energy transferred; power (W) is the rate — the same work done more quickly requires more power
- Forgetting to convert time for power: P = W ÷ t requires time in seconds — always convert minutes to seconds before calculating power
Memory Aid
- "Work = Force × Shift": The 's' in W = Fs stands for the shift (displacement) in the direction of the force. If there's no shift in the force direction, there's no work.
- The "W.F.S." triangle: Draw a triangle with W at the top, F and s at the bottom corners. Cover the one you want: W = F × s; F = W ÷ s; s = W ÷ F.
- Joule = Newton-metre: Remember "J = Nm" — 1 Joule is 1 Newton acting through 1 metre. This link between energy and force-times-distance is the whole point of work!