CP7 · Energy — forces doing work · Topic 8 — Energy - Forces doing work | GCSE Edexcel Combined Science

Revise the key ideas

When a force moves an object through a distance in its direction, it transfers energy mechanically. This energy transfer is called work done and is measured in joules (J).
For a constant force acting along the movement, work done = force × distance moved in the direction of the force, E = Fd. Use force in N and distance in m; 1 J = 1 N m.
Both force and displacement are along the same direction.
Pushing a crate with 20 N through 3 m in the force direction transfers 60 J. Do not multiply by distance in an unrelated direction.
If an object does not move, no mechanical work is done on it by the supporting force, even though a person's muscles may still transfer energy internally.
A force at right angles to an object’s movement does no work on it. For example, the inward force on an object moving in a circle at constant speed changes its direction but not its speed.
Work done against friction transfers energy to thermal stores of the surfaces and surroundings. This can be useful in brakes but unwanted in machinery.
Measure force using a newton meter and displacement using a ruler or tape. A constant-force experiment needs the force direction aligned with the measured movement.

Energy can be stored kinetically, thermally, chemically, gravitationally, elastically, magnetically, electrostatically or nuclearly. Transfer pathways include mechanical work, electrical transfer, heating and radiation.
Energy is conserved: it cannot be created or destroyed. In a closed system, transfers move energy between stores or places, but the total stays the same.
A motor lifting a load transfers energy electrically to the motor and mechanically to the load’s gravitational potential store. Some energy spreads to thermal stores in the surroundings (is dissipated).
Useful gravitational energy plus unwanted transfers equals input energy.
When a falling object speeds up, its gravitational potential store decreases while its kinetic store increases. Drag adds thermal transfers, so the kinetic gain may be smaller than the gravitational decrease.
Dissipation spreads energy into less useful thermal stores; sound can carry energy away. “Wasted” describes usefulness for the task, not failure of conservation.
Energy-flow diagrams identify input, useful output and unwanted transfers. For quantitative diagrams, the total output must equal input.

Change in gravitational potential energy = mgΔh. Use mass in kg, gravitational field strength in N/kg and vertical height change in m to calculate energy in J.
A 5 kg mass raised 2 m where g = 10 N/kg gains 100 J. The change depends on vertical rise, not the length of an inclined path.
Kinetic energy = ½mv², using mass in kg and speed in m/s. Doubling mass doubles kinetic energy; doubling speed quadruples it.
A 2 kg object at 4 m/s has kinetic energy ½ × 2 × 16 = 16 J. To find speed, rearrange v = √(2E/m).
If all lost GPE becomes kinetic energy, mgΔh = ½mv². The mass cancels; ignoring drag, the speed gained from a height depends on g and height.
Conservation connects the two energy stores.
To stop an object, its kinetic energy must be transferred. For a constant braking-force magnitude, Fd = ½mv²; larger speed increases required stopping distance strongly.
For a load raised at constant speed, lifting force equals its weight and work done = weight × vertical distance = mgΔh. Acceleration or other losses may change input requirements.

Power is the rate of energy transfer or work done: P = E/t. Power is measured in watts (W); 1 W = 1 J/s.
A machine doing 600 J of work in 3 s has power 200 W. Two machines can do equal work but have different power if they take different times.
Rearrange to E = Pt and t = E/P. Convert minutes to seconds and kilowatts to watts before using SI units.
To investigate power while climbing stairs, measure mass, vertical height gained and time; calculate useful power = mgΔh/t. Use safe walking/running conditions and repeat.
Use vertical height, not the sloping staircase length.
To investigate lifting power, time raising a known weight through a measured vertical height. Control the load and height when comparing machines or people.
Measured useful lifting power may be smaller than total input power because energy is also transferred to surroundings. State whether a calculation uses input or useful output.

Efficiency = useful energy output ÷ total energy input. The same ratio applies to useful power ÷ input power when comparing the same operating interval.
Efficiency is a fraction from 0 to 1 or a percentage from 0% to 100%. A 60% efficient motor gives 60 J useful output for every 100 J input.
Both examples have efficiency 60%; energy totals balance.
Useful output = efficiency fraction × input. Total input = useful output ÷ efficiency fraction; use 0.6 rather than 60 in these equations for 60%.
Lubrication can reduce frictional heating. Insulation reduces unwanted heating of the surroundings; the improvement depends on the intended task.
For a kettle, heating the water is useful, while heating the room is unwanted. For a lamp, light is useful while most heating is unwanted. Identify the task before classifying transfers.