Unit 4, lesson 23. About 12 minutes.

Energy in your body

How long could one banana keep you going?

Play

Getting the experiment ready…

Explain

Food is fuel for your body.

Running uses more than sleeping.

Food contains chemical energy. Your body releases this energy slowly, and uses it for everything you do.

Even when you are asleep, your body uses energy to breathe, pump blood, and keep warm.

Food labels show energy in kilojoules (kJ) or calories. Pick a food and an activity to compare them.

Time = energy / power. Food energy is given in kJ, and body power is in watts (J/s).

A sleeping adult uses about 100 W, and someone running hard about 700 W or more.

Worked example: a 400 kJ banana while walking at 300 W lasts 400 000 / 300 ≈ 1300 s, or about 22 minutes.

Food stores energy
Your body releases it
You move and stay warm

Want the full story?

These figures are rough, for an adult. They vary a great deal by person, age and activity. Bodies waste around three-quarters of food energy as heat when muscles work, and the power values shown already include that. All foods also matter for health, not only for energy.

The Formula

time = energy ÷ power

t = \frac{E}{P}

$t$: the time the energy lasts, measured in seconds (s)
$E$: the energy in the food, measured in joules (J)
$P$: the rate your body uses energy, measured in watts (W)

Try the numbers

Food energy: 400 kJBody power: 250 W

Time it lasts: 26.67 minutes

Try it at home

You need some food packets, a calculator.

You need: some food packets, a calculator.

Look at a food box. Can you find the numbers?

Find the energy figure on a food label, in kJ.
Divide by 0.3 to get the seconds of walking it would fuel.
Divide by 60 to get the minutes.

What's happening: dividing kJ by the power in kW gives seconds. Walking uses about 0.3 kW.

Physics is life

Your body

Your body burns food like fuel.

Your brain alone uses about 20 W, which is around a fifth of your resting energy.

Daily life

Food labels show energy.

Children need roughly 6000–9000 kJ a day, depending on age and how active they are.

Nature

Bears store fat for winter.

Hibernating animals slow their bodies down so that stored fat lasts all winter.

Space

Astronauts need food in space.

Astronauts eat carefully planned food, since working in space uses a lot of energy.

Quick check

Question 1 of 3

Where does your body get energy?

Words to know

Kilojoule (kJ): One thousand joules. Food labels use it.

Kilojoule (kJ): One thousand joules. Food labels use it.
Calorie: An older unit of energy. One food calorie is about 4.2 kilojoules.
Energy: What makes things able to happen. It is measured in joules.

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Momentum