1. What does power measure?
- Amount of force stored
- Rate of energy transfer
- Energy per metre
Check the answer and reasoning
Rate of energy transfer
One watt is one joule transferred each second.
Physics / Foundations
Energy, work and power
Track transfers rather than treating energy as a force.
Learning objectives
- Identify kinetic and gravitational energy stores.
- Calculate work done by a constant force.
- Explain why power depends on time.
The big idea
Energy is transferred when work is done, by heating, electrically or by radiation. A moving object has kinetic energy; lifting an object increases its gravitational potential energy.
Total energy is conserved. Useful energy may spread into the surroundings as thermal energy, making it less available for the intended task. Power measures how rapidly energy is transferred.
1. Tell an energy-transfer story
When you lift a shopping bag, your muscles transfer energy to its gravitational store. When you release it, gravitational potential energy can transfer to kinetic energy. Energy is an accounting quantity measured in joules, not a substance that must flow through a visible pipe.
2. Useful does not mean conserved
A rolling toy slows because some mechanical energy transfers to thermal stores and sound. Total energy still balances when the surroundings are included. We call part of a transfer useful according to the task, such as raising a load, rather than according to a new conservation law.
3. Separate energy from power
Lifting a 2 kg bag through 1 m requires about 20 J where g = 10 N/kg. Doing it in 2 s gives average useful power 10 W; doing it in 4 s gives 5 W. The energy increase is the same, but the rate of transfer changes.
A method you can reuse
- Name the initial and final energy stores.
- State which surroundings must be included.
- Calculate the transfer in joules.
- Divide by time only when power is requested.
Where has the energy gone when a toy stops?
It has largely transferred to internal energy of the toy and surroundings, with some sound. Stopping does not destroy energy.
Equations and relationships
Change in gravitational potential energy near Earth.
Average power is energy transferred per unit time.
Worked example
A 2 kg mass is lifted 3 m. Take g = 10 N/kg. What energy is gained?
Follow the solution
2 × 10 × 3 = 60 J of gravitational potential energy.
Investigate and explain
Release a simulated skater from two heights, keeping the track and friction settings fixed; compare bottom speeds.
Use the energy bars to track transfers, including thermal energy. Models simplify the surroundings; a missing visual effect is not evidence that its energy is absent in reality.
Check your understanding
2. A 2 kg mass is lifted 3 m. Take g = 10 N/kg. What energy is gained?
- 6 J
- 15 J
- 60 J
Check the answer and reasoning
60 J
2 × 10 × 3 = 60 J of gravitational potential energy.
3. What happens to energy when friction warms a surface?
- Energy transfers to thermal stores.
- Energy disappears.
- Energy becomes a force.
Check the answer and reasoning
Energy transfers to thermal stores.
Conservation includes the surroundings.