The shape of the lesson
By the end of the lesson, learners can
- Push, move, power: A constant force drags a crate at a steady speed against friction.
- Three relations to own: Match each to the situation: work for an energy transfer, power for a rate, efficiency for a ratio.
- Where the energy goes: Conservation of energy lets you track every joule from input to useful output and waste.
- Work: W = Fs cosθ, the component along the motion.
- Power: P = W / t = Fv.
- Efficiency: useful output / total input, never above 100%.
One picture every idea returns to
Work done = force × distance moved in the direction of the force, W = Fs cosθ. Energy is conserved: the total stays constant as it transfers between stores. Power is the rate of doing work, P = W / t, and for a constant force moving at speed v, P = Fv. Efficiency is the useful output divided by the total input.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Work, energy and power.. Energy is the currency of physics: never created or destroyed, only moved between stores and changed in form. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Push, move, power | A constant force drags a crate at a steady speed against friction. Project tab 1 of the lab (Work done and power) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Three relations to own | Match each to the situation: work for an energy transfer, power for a rate, efficiency for a ratio. | Pairs |
| 21 to 30 min | Teach and check: Where the energy goes | Conservation of energy lets you track every joule from input to useful output and waste. | Mix-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Where the energy goes, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Two slips recur. | Work is zero when the force is perpendicular to the motion, so carrying a bag horizontally does no work against gravity. And when a force acts at an angle, you must include cosθ, using only the component along the motion. Efficiency can never exceed 100%; if your answer does, you have swapped useful and total. |
Work, energy and power: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
State what is meant by the work done by a force, and give its SI unit.
A 25 N force drags a box 6.0 m along the floor in the direction of the force, taking 3.0 s. Find the work done and the average power.
A force of 80 N is applied at 25° to the horizontal and moves an object 5.0 m horizontally. Find the work done by the force.
A pump raises 300 kg of water through a height of 12 m every minute. Taking g = 9.8 m s⁻², find the useful output power.
An electric motor is supplied with 500 W and delivers 350 W of mechanical power. Find its efficiency.
A car travels at a steady 30 m s⁻¹ against a total resistive force of 600 N. Find the driving force and the power developed.
Answer key and marking notes
- Work done = force × distance moved in the direction of the force ✓
- The SI unit is the joule (J) ✓
- Work = Fs = 25 × 6.0 = 150 J ✓
- Power = W / t = 150 / 3.0 = 50 W ✓
- Use the component along the motion: W = Fs cosθ ✓
- W = 80 × 5.0 × cos 25° = 80 × 5.0 × 0.906 ✓
- W = 363 J ✓
- Work each minute = mgh = 300 × 9.8 × 12 = 35 280 J ✓
- Power = work / time = 35 280 / 60 ✓
- Power = 588 W ✓
- Efficiency = useful / total = 350 / 500 ✓
- Efficiency = 0.70 = 70% ✓
- At constant speed the driving force balances the resistance, so it is 600 N ✓
- Power = Fv = 600 × 30 = 18 000 W = 18 kW ✓
20 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Work done by a force is the force multiplied by the distance moved:
- Ain the direction of the force
- Bat right angles to the force
- Cper second
- Dby the source
A 40 N force moves a box 5.0 m in its own direction. The work done is:
- A8.0 J
- B45 J
- C200 J
- D0.125 J
The SI unit of work and of energy is the:
- Awatt
- Bjoule
- Cnewton
- Dpascal
Power is defined as the:
- Atotal work done
- Bforce times distance
- Cwork per unit force
- Drate of doing work
A machine does 1200 J of work in 8.0 s. Its power is:
- A150 W
- B9600 W
- C0.0067 W
- D1208 W
A force of 250 N drives a vehicle at 12 m s⁻¹. The power developed is:
- A21 W
- B250 W
- C3.0 kW
- D3000 J
The watt is equivalent to:
- Aone newton per second
- Bone joule per second
- Cone joule per metre
- Done newton metre
A 60 W lamp emits 12 W as light. Its efficiency is:
- A48%
- B500%
- C72%
- D20%
Efficiency can never exceed 100% because:
- Athe useful output cannot exceed the total energy input
- Benergy is destroyed
- Cpower is always large
- Dwork is a vector
A force of 30 N acts at 60° to the motion as an object moves 4.0 m. The work done is:
- A120 J
- B104 J
- C60 J
- D15 J
A satellite moves in a circular orbit at constant speed. The work done on it by gravity each orbit is:
- Aequal to its weight times the circumference
- Bzero, because the force is perpendicular to the motion
- Cequal to its kinetic energy
- Dnegative
A crane raises a 500 kg load at a steady 0.40 m s⁻¹ (g = 9.8). Its useful output power is:
- A200 W
- B4.9 kW
- C1250 W
- D2.0 kW
A motor with input power 800 W is 75% efficient. Its useful output power is:
- A600 W
- B1067 W
- C200 W
- D60 W
A car of weight 12 kN climbs a slope at 8.0 m s⁻¹, rising 1.0 m for every 10 m travelled. The power against gravity is:
- A96 kW
- B1.5 kW
- C9.6 kW
- D12 kW
A box is pushed across a rough floor at constant speed. The work done by the push is:
- Azero
- Bequal to the work done against friction
- Cstored as kinetic energy
- Dgreater than the friction work
Which quantity is a measure of how quickly energy is transferred?
- Awork
- Bforce
- Cefficiency
- Dpower
A pump lifts 50 kg of water per second through 4.0 m (g = 9.8). The minimum power needed is:
- A2.0 kW
- B200 W
- C490 W
- D1.25 kW
Doubling the force on an object while keeping its speed the same changes the power to:
- Ahalf
- Bunchanged
- Cdouble
- Dfour times
How much work is needed to lift a 2.0 kg book 1.5 m at constant speed (g = 9.8)?
- A3.0 J
- B29 J
- C13 J
- D20 J
If a 100 W and a 60 W lamp run for the same time, the 100 W lamp transfers:
- Aless energy
- Bthe same energy
- Cno energy
- Dmore energy
Quiz key
W = Fs cosθ uses the distance in the direction of the force.
W = Fs = 40 × 5.0 = 200 J.
Work and energy are measured in joules (J).
Power = work done per unit time, P = W/t.
P = W/t = 1200/8.0 = 150 W.
P = Fv = 250 × 12 = 3000 W = 3.0 kW.
1 W = 1 J s⁻¹.
Efficiency = useful/total = 12/60 = 20%.
Energy is conserved, so output cannot exceed input.
W = Fs cosθ = 30 × 4.0 × cos 60° = 60 J.
The gravitational force is perpendicular to the velocity, so it does no work.
P = mgv = 500 × 9.8 × 0.40 = 1960 W ≈ 2.0 kW.
Output = efficiency × input = 0.75 × 800 = 600 W.
Vertical speed = 8.0/10 = 0.80 m s⁻¹; P = (12000)(0.80) = 9600 W.
At constant speed the kinetic energy is unchanged, so the push work equals the friction work (becoming heat).
Power is the rate of energy transfer.
P = (mg/t)h... = (50 × 9.8 × 4.0) per second = 1960 W ≈ 2.0 kW.
P = Fv, so doubling F at the same v doubles the power.
W = mgh = 2.0 × 9.8 × 1.5 = 29 J.
Energy = power × time, so the higher-power lamp transfers more energy in the same time.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.