Work vs Power
Work, in physics, is force applied through a distance — the amount of energy transferred when a force moves an object. Power is the rate at which work is done — the same job completed in less time means more power. Work measures "how much"; power measures "how fast."
Last reviewed on 2026-04-27.
Quick Comparison
| Aspect | Work | Power |
|---|---|---|
| What it is | Energy transferred by a force over a distance | Rate of doing work (energy per unit time) |
| Formula | W = F × d (when force is along distance) | P = W/t |
| SI unit | Joule (J) | Watt (W) = J/s |
| Includes time? | No | Yes |
| Same work, different time | Same number of joules | Different watts depending on how fast |
| Example | Lifting 10 kg by 1 m = ~98 J of work | Lifting it in 1 s vs 10 s = 98 W vs 9.8 W |
| Common everyday units | Foot-pounds, calories | Watts, kilowatts, horsepower |
Key Differences
1. Energy versus rate
Work tells you the total energy transferred. Move a box 5 m with a force of 20 N along the direction of motion, and you've done 100 J of work — regardless of how long it took.
Power tells you how fast that energy was delivered. Doing 100 J in 1 s requires 100 W. Doing the same 100 J in 10 s requires only 10 W. Same total work, very different power demands.
2. Units
Work uses joules (or kilowatt-hours, calories, foot-pounds in different contexts). 1 J = 1 N·m = energy needed to lift roughly 100 g by 1 m.
Power uses watts (and kilowatts, megawatts, horsepower). 1 W = 1 J/s. A 60 W light bulb consumes 60 J of electrical energy every second.
3. When time matters
A weightlifter doing a slow squat does similar work to one doing a fast squat with the same weight and depth.
But the fast squat requires much higher power — that's why explosive lifts feel different and develop different muscle qualities than slow grinds.
4. Energy bills versus appliance ratings
Your electricity bill measures total work done by electrical energy — usually in kilowatt-hours (kWh). 1 kWh = 1 kW × 1 hour = 3.6 million joules.
Appliance ratings are usually power — a 1500 W kettle, a 50 W lamp. Multiply power by time to get the total work (and bill).
5. Cars: torque, horsepower, and you
Torque is rotational force; multiply it by angular displacement and you get rotational work.
Horsepower is rotational power. A car that produces high horsepower can complete the same amount of work (acceleration over a given distance) faster than a low-horsepower car of the same weight.
6. A negative work case
Work is positive when force is in the direction of motion, negative when force opposes motion.
Friction does negative work on a moving object — taking kinetic energy away. Brakes do negative work (and convert that work to heat).
When to Choose Each
Choose Work if:
- Calculating energy needs of any operation: lifting, pushing, accelerating.
- Determining how much fuel or food is required for a task.
- Engineering analysis of mechanical, electrical, or chemical systems.
Choose Power if:
- Sizing engines, motors, generators, and electrical equipment.
- Understanding why doing more in less time costs more energy capacity.
- Comparing the capability of devices, vehicles, and athletes.
Worked example
Two workers are asked to lift identical loads to a shelf 2 m up. Worker A finishes in 10 seconds; Worker B finishes in 30 seconds. They've done the same work — same force, same distance. But Worker A's power output was three times higher than Worker B's. Same job, three times the rate, three times the demands on muscle and metabolism.
Common Mistakes
- "Doing it faster is more work." It's more power, not more work. Work is energy transferred; rate doesn't change the total.
- "Watts and joules are the same." Different units. Watts are joules per second.
- "Holding something heavy is doing work." If the object isn't moving, no physics work is done — even though it feels like effort biologically.
- "Power is just for cars and electricity." Power applies to any energy transfer rate, including biological systems and economics.