Work and Efficiency (Qualitative)
At the 2032 Brisbane Olympics, engineers designing the new Gabba stadium must calculate exactly how many joules of work cranes do lifting 50,000 tonnes of steel — because wasted energy means wasted money and time.
Printable Worksheets
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Q1 · You push very hard on a wall for 5 minutes and don't move it. Are you "doing work" in the physics sense?
Q2 · An electric car is about 90% efficient; a petrol car is about 25%. What happens to the other 75% of the petrol car's energy?
● Know
- The physics definition of work (force causing movement)
- Efficiency = useful energy out ÷ total energy in × 100%
- No machine or device is 100% efficient
● Understand
- Why pushing a stationary wall does zero physics work
- Why all real machines waste some energy as heat or sound
- How to improve efficiency (reduce friction, better insulation, better components)
● Can do
- Identify situations where physics work IS and IS NOT done
- Calculate efficiency as a percentage given energy values
- Distinguish everyday "work" from the physics definition
Grip a heavy backpack and hold it still for 30 seconds — your arm aches and your muscles burn, yet in the physics sense you have done zero work. Now take one step forward with that same bag: your muscles do exactly the same effort, but this time physics says you have done work. The difference? Movement in the direction of the force — that is the only thing physics cares about.
Physics work is done when a force causes an object to move in the direction of the force. If no movement occurs → no work is done.
Key examples:
- Work IS done: lifting a bag, pushing a trolley across a supermarket, a hammer hitting a nail.
- Work is NOT done (in physics): holding a heavy box steady without moving (no displacement), pushing a wall that doesn't move, sitting still.
Carrying a box horizontally while walking — technically, the upward force and the horizontal movement are perpendicular, so no work is done by the lifting force against gravity. This surprises most people!
The key insight: work = energy transfer. If you do physics work on an object, you've transferred energy to it.
Every time energy is transformed, some is lost to less useful forms — usually heat or sound. This is why no machine is ever 100% efficient.
Efficiency (%) = (useful energy output ÷ total energy input) × 100
| Device | Efficiency | Where energy is wasted |
|---|---|---|
| LED light bulb | ~90% | ~10% as heat |
| Electric motor | ~90% | ~10% as heat and sound |
| Solar panel (PV) | ~20% | ~80% as heat (reflected or absorbed) |
| Petrol car engine | ~25% | ~75% as heat (exhaust, coolant, friction) |
| Human body (cycling) | ~25% | ~75% as heat (body warmth, sound) |
Improving efficiency:
- Reduce friction (lubrication, smooth surfaces)
- Insulate to keep heat where it is needed
- Use more efficient components (LED vs incandescent globe)
- Recapture waste heat (heat exchangers in industry)
Australia's government Energy Star rating system labels fridges, washing machines, TVs and other appliances with a star rating — more stars = more efficient = less waste energy per useful task.
A machine can have MA > 1 but can never have efficiency > 100%. If a machine claims to give more energy out than in, the measurement is wrong — or someone is confused about what counts as "useful".
Perpetual motion machines are impossible. A machine that claims to run forever without any energy input would need 100%+ efficiency, which violates conservation of energy.
One interesting case: heat pumps (reverse-cycle air conditioners) appear to be >100% efficient because they move heat from outside to inside rather than generating heat from electricity. They can deliver 3–5 units of heat energy for every 1 unit of electrical energy. But they are NOT violating energy conservation — they are harvesting free thermal energy from the environment. Australia's NABERS building rating system recognises heat pump efficiency in office buildings.
Australian energy efficiency highlights:
- Energy Star rating system — 1 to 6 stars on appliances (fridges, washing machines, TVs)
- NABERS rating for commercial buildings (1–6 stars) — major CBD buildings in Sydney and Melbourne are rated
- Electric vehicles — ~90% efficient vs ~25% for petrol; less waste heat per kilometre
Wrong: "I worked hard, so I did lots of physics work." Physics work requires movement. Trying hard without producing displacement does zero physics work, even though it feels exhausting.
Right: Physics work = force × displacement. Effort without movement = zero physics work. Everyday "hard work" and physics work are different concepts.
Wrong: "More efficient machines create energy." No machine creates energy. Higher efficiency means less is wasted — the total energy in always equals total energy out (as useful + waste). Energy is only ever converted, never created.
Right: Efficiency measures how much useful energy you get from the input. More efficient = less waste. Total energy is always conserved.
Wrong: "Carrying a heavy bag across a room is hard work, so lots of physics work is done on the bag." The upward force holding the bag and the horizontal movement are perpendicular — so no work is done by the supporting force. Physics work is direction-specific.
Right: Only force that acts in the direction of movement does work. Holding a bag up while moving horizontally does no work against gravity (though your muscles feel it).
You push very hard against a solid brick wall for 5 minutes and it doesn't move at all. In everyday life, you'd say that was hard "work". Predict: in physics terms, how much work did you do ON THE WALL? And where did all your body's energy actually go?
How close was your prediction?
The hook at the start of this lesson said a petrol car converts only about 25% of its fuel energy into motion. Now you know exactly where that other 75% goes — and how physicists define "work" to calculate it precisely.
Explain why pushing a stationary wall does zero physics work even if you're exhausted, then connect this to why an engine wasting 75% of its energy as heat is not "doing work" in the useful sense. Use the words force, displacement and work at least once each.
Q1. Explain the physics definition of work. Describe two situations where you exert a force — one where physics work IS done and one where it is NOT. (3 marks)
Q2. A hair dryer uses 1200 J of electrical energy. 1080 J becomes useful heat for drying hair; the rest is sound and wasted heat. Calculate the efficiency of the hair dryer. (2 marks)
Q3. Why is 100% efficiency impossible in a real machine? Use the concept of energy conservation in your answer, and give two strategies engineers use to improve efficiency. (4 marks)
Answers
▾MCQ 1
A — Physics work requires both a force and displacement in the direction of the force. Pushing a wall without movement = no displacement = zero work on the wall. Potential energy or balanced forces don't themselves constitute work being done.
MCQ 2
B — Efficiency = (useful energy out ÷ total energy in) × 100%. It is not about speed, total energy, or run time. The result is a percentage: 100% would mean no energy is wasted (impossible in practice).
MCQ 3
B — Efficiency = (900 ÷ 1000) × 100 = 90%. The remaining 10% (100 J) was wasted as heat or sound. 110% would violate conservation of energy.
MCQ 4
B — Friction, air resistance, and other factors always produce some heat or sound during energy transformation. Conservation of energy is never violated — total energy in = useful out + waste out. Engineers minimise waste but cannot eliminate it entirely.
MCQ 5
C — Work done on the wall = zero because the wall doesn't move (no displacement). Note: the person exerts metabolic energy (gets tired) but this is converted to body heat, not transferred to the wall as useful work.
Short Answer 1
Model answer: Physics work is done when a force causes an object to move in the direction of the force — work = energy transferred by a force through a displacement. Example where work IS done: lifting your schoolbag from the ground (upward force, upward movement). Example where work is NOT done: pushing against a brick wall that doesn't move (force applied, but zero displacement means zero work on the wall).
Short Answer 2
Model answer: Efficiency = (1080 ÷ 1200) × 100 = 90%. The remaining 10% (120 J) is wasted as sound and heat that doesn't help dry the hair.
Short Answer 3
Model answer: 100% efficiency is impossible because the law of conservation of energy states that energy cannot be created or destroyed — only transformed. Every transformation involves some energy being converted to heat or sound (less useful forms), so the useful output is always less than the total input. Engineers improve efficiency by: (1) reducing friction — e.g. lubricating moving parts so less energy is lost to heat; (2) using better-insulated components — e.g. double-glazed windows keep heat from escaping buildings; other valid answers include heat exchangers, LED lighting, or aerodynamic design.