Newton’s Three Laws of Motion, Explained With Everyday Examples
Newton’s first, second and third laws of motion explained simply, with everyday examples, the F = ma formula, worked problems and common misconceptions.
Work, Energy and Power Formulas
Work, energy and power explained: W = Fd, P = W/t, units like joules and watts, efficiency, and worked examples from lifting boxes to climbing stairs.
Key takeaways
- Work is force times distance moved in the direction of the force: W = Fd.
- Power is how fast work is done: P = W ÷ t, measured in watts.
- Efficiency = useful energy output ÷ total energy input.
On this page
In physics, “work” has a precise meaning: energy transferred when a force moves something. Power tells you how quickly that happens.
Work
W = F × d
- F = force (N)
- d = distance moved in the direction of the force (m)
- W in joules (J)
Holding a heavy bag still does no work in the physics sense, because nothing moves. If the force is at an angle, use W = F d cos θ.
Energy
Work transfers energy. Lifting a box gives it gravitational potential energy; pushing a cart gives it kinetic energy. See kinetic vs potential energy.
Power
P = W ÷ t
- P in watts (W); 1 W = 1 J/s
Two people climbing the same stairs do the same work, but the faster one is more powerful.
Worked example
A 60 kg person climbs 3 m of stairs in 4 s. Work = mgh = 60 × 9.81 × 3 ≈ 1,766 J. Power = 1,766 ÷ 4 ≈ 441 W.
Efficiency
Efficiency = useful output ÷ total input × 100%. No machine is 100% efficient; some energy becomes heat, often through friction.
Electrical power
In circuits, P = V × I; see our Ohm’s law calculator.
Summary
| Quantity | Formula | Unit |
|---|---|---|
| Work | W = Fd | Joule (J) |
| Power | P = W/t | Watt (W) |
| Efficiency | useful ÷ total | % |
Frequently asked questions
What is the difference between work and power?
Work is energy transferred; power is the rate of transfer.
What is a kilowatt-hour?
A unit of energy: 1 kW for one hour = 3.6 million joules, used on electricity bills.
Can work be negative?
Yes, when the force opposes the motion, like friction slowing an object.
Sources
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