Energy, Work and Power

Energy that gets things done

In everyday speech, "work" means a job or effort. In physics it has a precise meaning, and it is tightly linked to energy and power. Master these three and you can calculate how machines, engines, and even your own body perform. They build on the idea of energy from the many forms of energy.

What is work?

In physics, work is done whenever a force moves an object through a distance. Push a box across the floor and you do work on it. The formula is:

work done = force × distance W = F × d

• Work (W) is measured in joules (J).
• Force (F) is measured in newtons (N).
• Distance (d) is measured in metres (m), in the direction of the force.

One joule is the work done when a force of one newton moves an object one metre.

A key point: if nothing moves, no work is done — no matter how hard you push. Straining against a locked wall transfers no mechanical work to it, because the distance moved is zero.

Work transfers energy

Here is the deep idea that ties everything together: doing work on an object transfers energy to it.

When you lift a book onto a shelf, you do work against gravity, and that work becomes gravitational potential energy stored in the book. When an engine pushes a car forward, the work it does becomes kinetic energy of motion. Because of this, work done = energy transferred, and both are measured in joules. The forces involved here obey Newton's laws of motion.

What is power?

Two cranes lift identical loads to the same height. They do the same work. But if one does it in 5 seconds and the other in 20, the faster crane is more powerful.

Power is the rate of doing work, or of transferring energy:

power = energy transferred ÷ time P = E ÷ t (equivalently P = W ÷ t)

• Power (P) is measured in watts (W).
• One watt equals one joule per second.

A 60 W light bulb transfers 60 joules of energy every second. A 2000 W kettle transfers energy more than 30 times faster, which is why it boils water so quickly.

Efficiency

When a device transfers energy, not all of it ends up doing the useful job. Some is dissipated, usually as wasted heat (and sometimes sound), due to friction and resistance.

Efficiency measures how much of the input energy becomes useful output:

efficiency = (useful energy output ÷ total energy input) × 100%

A modern LED bulb might be 90% efficient — most input becomes light. An old filament bulb was only about 5% efficient, wasting the rest as heat. Because energy is conserved, the wasted energy is never destroyed; it simply spreads out as low-grade heat, which is why no real machine reaches 100% efficiency.

Worked example 1: work done

A weightlifter raises a 60 kg barbell to a height of 2 metres. How much work is done?

First find the weight (the force of gravity), using about 10 N/kg:

force = 60 kg × 10 N/kg = 600 N

Then apply the work formula:

work = force × distance = 600 N × 2 m = 1200 J

The lifter transfers 1200 joules of energy, now stored as gravitational potential energy in the raised barbell.

Worked example 2: power

Suppose that same lift took 3 seconds. What was the lifter's power output?

power = energy ÷ time = 1200 J ÷ 3 s = 400 W

If a stronger athlete lifted the same bar in just 2 seconds, their power would be 1200 ÷ 2 = 600 W — more powerful, even though the work done is identical.

Try it yourself! 🔬

Measure your own power climbing the stairs.

1. Find your mass in kilograms and a flight of stairs. Measure the total vertical height you will climb (height of one step × number of steps).
2. Calculate your weight: mass × 10 N/kg. This is the force.
3. Work done = weight × height climbed (in joules).
4. Time yourself running up the stairs in seconds. Then power = work ÷ time.

For example, a 50 kg student (weight 500 N) climbing 4 m in 5 s does 500 × 4 = 2000 J of work, giving 2000 ÷ 5 = 400 W of power — comparable to a bright old light bulb. Try walking, then running, and watch your power output rise.