This mini-lesson walks you through the whole of Edexcel Topic 8 — Energy: forces doing work. The heart of it is the idea that a force doing work is one of the ways energy gets shifted between stores, and that power measures how fast that happens.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
8.1 · Systems
When a system changes, stores change
A system is just the object (or group of objects) you choose to study. When the system changes — something speeds up, gets lifted, gets warmer — energy moves between its stores.
A ball is kicked → its kinetic store fills.
A crate is dragged up a ramp → its gravitational store fills.
Brakes grip a wheel → the thermal store of the brakes fills.
Describe the change, not just the labels. Edexcel wants you to say which store empties and which fills — e.g. "the chemical store of the fuel empties and the kinetic store of the car fills."
Quick check
Describe the store change
?A cyclist freewheels from a standstill down a hill, getting faster and faster. Which description of the store change is best?
8.2 · Transfer diagrams
Drawing energy transfers
An energy-transfer diagram shows stores as boxes and the transfer as a labelled arrow. Label the arrow with the pathway doing the shifting.
An electric winch: energy shifts electrically into the motor, then the motor's force does work filling the load's gravitational store.
Closed system (8.3): if you draw a box around the whole set-up so nothing crosses the boundary, the total energy never changes — there is no net change. Energy only moves around inside.
Sort it
How is the energy shifted?
Edexcel lists three ways a system's energy can be changed: work done by forces, electrical working, and heating. Tap the right one.
8.5–8.6 · The big one
Work done by a force
When a force makes something move, the force does work — and the work done equals the energy transferred. They are measured in the same unit, the joule.
W = F × dwork done (J) = force (N) × distance moved in the direction of the force (m)
The force F pushes the box a distance d in the direction of the push. Work done W = F × d fills the box's kinetic (and thermal, via friction) store.
Misconception — no movement, no work. If the force does not move the object along its line of action, no work is done. Holding a heavy bag still does zero work on it (d = 0), even though your arm gets tired!
Worked example
A trolley is pushed with a steady force of 25 N for 4 m along the floor.
W = F × d = 25 × 4 = 100 J — so 100 J is transferred.
Calculate
Your turn — work done
1A removal worker pushes a sofa across a floor with a horizontal force of 150 N, moving it 6 m. Calculate the work done on the sofa.
J
Hint: W = F × d = 150 × 6.
Calculate
Rearranging W = F × d
2A cyclist does 4800 J of work pedalling against a steady resistive force of 80 N along a flat road. How far did the bike travel?
m
Hint: rearrange to d = W ÷ F = 4800 ÷ 80.
8.8 · Lifting
Work against gravity → gravitational store
Lift an object and your force does work against gravity. That work is stored in its gravitational store:
ΔGPE = m × g × Δhchange in g.p.e. (J) = mass (kg) × gravitational field strength (N/kg) × change in vertical height (m)
Δh is vertical height only. Carry a box 10 m along a flat corridor and its gravitational store doesn't change — Δh = 0. Only the rise counts.
Worked example
A 4 kg toolbox is lifted 1.5 m onto a van shelf (g = 10 N/kg).
ΔGPE = 4 × 10 × 1.5 = 60 J
Calculate
Your turn — gravitational store
3A warehouse lift raises a 250 kg pallet by 3 m. Using g = 10 N/kg, how much energy is transferred to its gravitational store?
J
Hint: ΔGPE = 250 × 10 × 3.
8.9 · Moving
Kinetic store of a moving object
Work done to speed something up fills its kinetic store. It depends on mass and, much more strongly, on speed:
KE = ½ × m × v²kinetic energy (J) = ½ × mass (kg) × (speed)² ((m/s)²)
Speed is squared, so a van going twice as fast has four times the kinetic energy — and needs four times the work to stop it.
Worked example
A 1500 kg van travels at 12 m/s.
KE = ½ × 1500 × 12² = ½ × 1500 × 144 = 108 000 J (108 kJ)
Calculate
Your turn — kinetic store
4A 0.4 kg football is kicked so it moves at 20 m/s. Calculate the energy in its kinetic store.
J
Hint: KE = ½ × 0.4 × 20². (20² = 400)
8.10–8.11 · Wasted energy
Energy dissipated to less useful stores
In every real change, some energy is dissipated — spread out into stores that are less useful to us, almost always by heating the surroundings.
Edexcel singles out mechanical processes: friction between moving parts causes a rise in temperature, so energy is dissipated by heating the surroundings and can't be recovered.
Drag a box across a rough floor and the useful kinetic store is steadily dissipated into the thermal store of the floor — spread out and unrecoverable.Quick check
Where does it go?
?A child whizzes down a metal slide but reaches the bottom slower than a frictionless calculation predicts. Where has the "missing" energy gone?
8.12–8.14 · The other big one
Power — how fast energy is transferred
Power is defined as the rate at which energy is transferred, i.e. the rate of doing work. Same energy, less time → more power.
P = E ÷ t = W ÷ tpower (W) = energy transferred / work done (J) ÷ time taken (s)
One watt is one joule per second (1 W = 1 J/s).
Both motors transfer the same 1200 J to the load's gravitational store. Motor B does it in a third of the time, so it is three times more powerful.
Misconception — power ≠ amount of energy. Power tells you how fast energy is transferred, not how much. Two machines can transfer the same energy; the one that does it in less time has the higher power.
Calculate
Your turn — power (energy ÷ time)
5A kitchen blender transfers 9000 J of energy in 30 s. Calculate its power.
W
Hint: P = E ÷ t = 9000 ÷ 30.
Calculate
Your turn — power (work ÷ time)
6Climbing the stairs, a student does 1500 J of work against gravity in 5 s. Calculate the student's useful power output.
W
Hint: P = W ÷ t = 1500 ÷ 5. (This is the Edexcel "power up the stairs" practical!)
8.15 · Efficiency
Useful vs total — efficiency
Because some energy is always dissipated, only part of the input ends up where you want it. Efficiency is the fraction that is useful:
efficiency = useful energy transferred ÷ total energy supplied(× 100 for a percentage) — it has no units and is always less than 1
An electric drill: of every 500 J supplied, 300 J usefully turns the bit and 200 J is dissipated by heating.
That drill's efficiency = 300 ÷ 500 = 0.6 = 60%. Efficiency can never be more than 1 (100%) — that would mean creating energy.
Calculate
Your turn — efficiency
7A crane is supplied with 40 000 J and usefully transfers 26 000 J to the gravitational store of a girder. Calculate its efficiency as a percentage.
%
Hint: (26 000 ÷ 40 000) × 100.
Match it
Equation ↔ what it finds
Tap an equation, then tap the quantity it calculates.
Quick check
Power or energy?
?Two electric hoists each lift the same crate to the same shelf, transferring 8000 J. Hoist X takes 10 s; hoist Y takes 40 s. Which statement is correct?
Recap
Topic 8 in a nutshell
Work done: W = F × d (energy transferred = work done, both in J)
Gravitational store: ΔGPE = m × g × Δh
Kinetic store: KE = ½ × m × v²
Power: P = E ÷ t = W ÷ t (1 W = 1 J/s)
Efficiency: useful ÷ total (no units, < 1)
Key ideas: closed system → no net change; energy is dissipated to less useful stores (heating surroundings).
You've covered the whole of Edexcel Topic 8 — Energy: forces doing work. Press Finish to see your score.
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