Work, energy, and power
Key ideas
Work is energy transferred by a force acting through a distance:
Power is how fast that energy is transferred:
The energy forms you track at Level 2:
| Energy | Formula | When it appears |
|---|---|---|
| Kinetic | anything moving | |
| Gravitational potential | anything raised or lowered | |
| Elastic potential | stretched/compressed springs |
Conservation of energy: energy is never created or destroyed, only transformed. In an ideal system (no friction), total energy at the start = total energy at the end. With friction, the "missing" energy has become heat and sound — and saying exactly that is often the Merit/Excellence step.
A kg box is lifted m onto a shelf in s at steady speed. Find the work done and the power.
Step 1 — Work done against gravity
At steady speed the lifting force equals the weight, :
Step 2 — Power = work ÷ time
A kg pumpkin drops from a m wall. How fast is it moving just before it lands (ignore air resistance)?
Step 1 — Name the energy transformation
Gravitational potential energy at the top becomes kinetic energy at the bottom:
Step 2 — Substitute and solve for
Practice question
A N student runs up a m high flight of stairs in s. What's their power output against gravity?
Worked solution: J; W.
Test yourself
Practice by grade
One question each at Achieved, Merit and Excellence. Have a go, then compare with the model answer.
A kg object is moving at m s⁻¹.
Calculate its kinetic energy.
A kg ball is dropped from a height of m.
Using energy, calculate its speed just before it hits the ground (ignore air resistance).
A kg box slides from rest down a ramp m high, reaching the bottom at m s⁻¹ — slower than a frictionless slide would give.
Calculate the amount of energy transferred to heat and sound by friction, using physics principles.