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The joule

Energy and power · level 4, lesson 1 of 6

A question to start

We measure distance in metres and mass in kilograms. What do we measure energy in, and how much is one unit? Is it the same on Mars?

The idea

Energy is measured in joules, written J, after James Prescott Joule, the English brewer and scientist who showed in the 1840s that work and heat are two forms of the same thing.

One joule is the energy needed to push with a force of 1 newton over a distance of 1 metre. It is a small amount:

  • Lifting a 100 g apple up by 1 m on Earth takes about 1 J.
  • A tennis serve gives the ball about 100 J.
  • A slice of bread holds about 300,000 J.

Because the numbers get big, we often use kilojoules (1 kJ = 1,000 J) and megajoules (1 MJ = 1,000,000 J).

1 m2.7 mEarth: 1 JMars: 1 J

The joule is the same everywhere. But gravity is not, so one joule lifts things higher on Mars:

1 joule is enough to Mars Earth
Lift a 100 g apple by 2.7 m 1.0 m
Lift a 1 kg bag by 27 cm 10 cm
Lift a 50 kg person by 5.4 mm 2.0 mm

Worked examples

1. Lifting a bag of rice. A 1 kg bag weighs 3.71 N on Mars. Lifting it 1 m takes \(3.71 \times 1 = 3.71\) J. On Earth, \(9.81\) J.

2. Kilojoules. 250,000 J is 250 kJ, or 0.25 MJ.

3. Climbing a flight of stairs. A 40 kg child climbs 3 m. On Earth: \(40 \times 9.81 \times 3 = 1{,}177\) J, about 1.2 kJ. On Mars: \(40 \times 3.71 \times 3 = 445\) J.

Try it

  1. What is the unit of energy?
  2. How much energy does it take to lift a 1 kg bag by 1 m on Earth? On Mars?
  3. Write 4,500 J in kJ.
  4. A 60 kg astronaut climbs a 5 m ladder on Mars. How much energy does it take (ignoring waste)?
Answers
  1. The joule, J.
  2. About 9.8 J on Earth, 3.7 J on Mars.
  3. 4.5 kJ.
  4. \(60 \times 3.71 \times 5 = 1{,}113\) J, about 1.1 kJ.

Common mistake

"Energy is measured in watts." Watts measure power, how fast energy is used. Energy is in joules (or kilowatt-hours). That is the next lesson.

Where it leads