Chapter 7 of 8

All 8 chapters
  1. 1Energy stores, readunderstood
  2. 2Energy transfers, readunderstood
  3. 3Energy is never used up, readunderstood
  4. 4Heat and temperature, readunderstood
  5. 5How heat travels, readunderstood
  6. 6Keeping warm and cool, readunderstood
  7. 7Energy from food, readunderstood
  8. 8Fuels and energy resources, readunderstood

S3.8.7 Chapter 7

Energy from food

Why am I so hungry after playing all day?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Compare the energy in foods using a table in kilojoules
  • Explain why active people need more food energy

Kofi plays football from morning until the sun is low. By the evening he is very hungry. His little sister sat in the shade all day and is only a little hungry.

Food is a chemical store. The energy in food is counted in kilojoules, written kJ. One kilojoule is 1000 joules. Your body takes the energy out of food and uses it.

Foods do not all hold the same amount. We compare them by the food energy in the same weight, usually 100 g. The numbers here are about right, not exact.

A table with two columns, food and kJ per 100 g. cooked rice: about 550 kJ. bread: about 1000 kJ. cooking oil: about 3700 kJ. banana: about 370 kJ. cooked beans: about 500 kJ. boiled egg: about 600 kJ. Labels: food, kJ per 100 g, cooked rice, 550 kJ, bread, 1000 kJ, cooking oil, 3700 kJ, banana, 370 kJ, cooked beans, 500 kJ, boiled egg, 600 kJ.foodkJ per 100 gcooked rice550 kJbread1000 kJcooking oil3700 kJbanana370 kJcooked beans500 kJboiled egg600 kJ
About how many kilojoules are in 100 g of each food.
A bar chart called kJ in 100 g. banana reaches 370, beans reaches 500, bread reaches 1000, oil reaches 3700. The scale goes from 0 to 4000 in steps of 1000. Labels: kJ in 100 g, banana, beans, bread, oil, 0, 1000, 2000, 3000, 4000.01000200030004000bananabeansbreadoilkJ in 100 g
Oil holds the most energy in the same weight. It is a fat. Fruit holds less, even when it tastes sweet.

Now the other side. Moving, thinking, growing and keeping warm all take energy out of your body. You are always using some, even when you sleep.

Draw it first

Draw how much energy an hour of each activity uses

  1. Three bars, one for each activity. The first bar is resting, 300 kJ, and is short. The second is walking, 1000 kJ, and is longer. The third is running, 2500 kJ, and is the longest. Labels: resting, walking, running, 300 kJ, 1000 kJ, 2500 kJ.resting300 kJ

    1 Draw a short bar for resting. In an hour it moves about 300 kJ out of the body.

  2. Three bars, one for each activity. The first bar is resting, 300 kJ, and is short. The second is walking, 1000 kJ, and is longer. The third is running, 2500 kJ, and is the longest. Labels: resting, walking, running, 300 kJ, 1000 kJ, 2500 kJ.resting300 kJwalking1000 kJ

    2 Draw a longer bar for walking. It moves about 1000 kJ in an hour.

  3. Three bars, one for each activity. The first bar is resting, 300 kJ, and is short. The second is walking, 1000 kJ, and is longer. The third is running, 2500 kJ, and is the longest. Labels: resting, walking, running, 300 kJ, 1000 kJ, 2500 kJ.resting300 kJwalking1000 kJrunning2500 kJ

    3 Draw the longest bar for running. It moves about 2500 kJ in an hour. The numbers are rough, for a grown person.

The more you move, the more energy leaves your body each hour. So active people need more food energy to put it back. Growing bodies also need food energy, to build new parts.

A band labelled chemical 100 J splits into a straight arrow labelled moving 25 J and a bent hatched arrow labelled heat 75 J. Labels: chemical 100 J, moving 25 J, heat 75 J.chemical 100 Jmoving 25 Jheat 75 J
Muscles turn only about a quarter of the food energy into movement. The rest is heat, and it keeps you warm.

Worked example

Tariq eats 200 g of cooked rice and 100 g of cooked beans. How much food energy is that?

  1. Rice: 200 g is 2 lots of 100 g. Each lot is about 550 kJ.
  2. Rice energy = 2 × 550 = 1100 kJ.
  3. Beans: 1 × 500 = 500 kJ.
  4. Total = 1100 + 500 = 1600 kJ.

Answer: about 1600 kJ

See it three ways

As a picture

A bar chart called kJ in the meal. The 200 g of rice reaches 1100. The 100 g of beans reaches 500. The scale goes from 0 to 1500 in steps of 500. Labels: kJ in the meal, rice, beans, 0, 500, 1000, 1500.050010001500ricebeanskJ in the meal

In words

To find the energy in a meal, work out how many lots of 100 g there are. Multiply by the kJ in 100 g. Do this for each food and add up.

In symbols

energy = (grams ÷ 100) × kJ per 100 g. For the rice: 2 × 550 = 1100 kJ.

Worked example

Joseph walks for 3 hours. Walking moves about 1000 kJ an hour. How much bread, in grams, holds the same energy? Bread has about 1000 kJ in 100 g.

  1. Energy moved = 3 × 1000 = 3000 kJ.
  2. Lots of 100 g of bread = 3000 ÷ 1000 = 3.
  3. Bread = 3 × 100 = 300 g.

Answer: 300 g

Try it

Feel your body move energy

You need: a clock or a friend who counts seconds, flat ground.

Be safe: Stop at once if you feel dizzy or any pain. Jog on flat ground with space around you.

  1. Sit still for two minutes. Press two fingers on your wrist, below the thumb, until you feel the pulse.
  2. Count the beats for 15 seconds. Multiply by 4. Write the number.
  3. Jog on the spot for one minute. Then count the pulse for 15 seconds again and multiply by 4.

What you will see: Your heart beats faster after jogging. Your muscles are moving more energy, so they need more oxygen and food energy. Your numbers will differ from ours.

If you cannot try it, this is what happens.
A table with two columns, when and per minute. resting: 72 beats. after jogging: 126 beats. Labels: when, per minute, resting, 72 beats, after jogging, 126 beats.whenper minuteresting72 beatsafter jogging126 beats

A wrong idea many people have

Only sweet foods have energy.

Why it feels right: Sugar gives a quick feeling of energy, so sweet things seem to be the energy foods.

The test that breaks it: Look at the table. Cooking oil is not sweet, yet it has about 3700 kJ in 100 g. Bread and beans are not sweet, and a banana is sweet but has only about 370 kJ.

What is true: Starch, sugar, fat and protein all hold energy. Fat holds the most in the same weight.

Key words

kilojoules
the units we count food energy in. We write kJ. 1 kJ is 1000 J.
food energy
the energy kept in a food, in its chemical store.

Back to your guess

The answer is C: Playing moves energy out of my body, so I need food to put it back.

Playing moves energy out of your body, into movement and heat. Food is a chemical store that puts energy back in.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. A table gives the kJ in 100 g of four foods. Which food has the most energy in the same weight?

    A table with two columns, food and kJ per 100 g. lentils: about 480 kJ. boiled potato: about 360 kJ. groundnuts: about 2400 kJ. milk: about 270 kJ. Labels: food, kJ per 100 g, lentils, 480 kJ, boiled potato, 360 kJ, groundnuts, 2400 kJ, milk, 270 kJ.foodkJ per 100 glentils480 kJboiled potato360 kJgroundnuts2400 kJmilk270 kJ
    Tap your answer.

    1. Not this one. You may have picked the first row. Compare every number, not only the top one. Go to S3.8.7. Try another.

    2. Not this one. Milk is a drink, so it can seem light. But 100 g of milk holds far less than 100 g of groundnuts. Compare the numbers. Go to S3.8.7. Try another.

    3. Not this one. A potato is bulky, but bulk is not energy. Read the kJ numbers. Go to S3.8.7. Try another.

    4. Yes, that is right. Groundnuts have about 2400 kJ in 100 g. That is the biggest number in the table.

    See the answer

    groundnuts

    Groundnuts have about 2400 kJ in 100 g. That is the biggest number in the table.

    • If you chose A: You may have picked the first row. Compare every number, not only the top one. Go to S3.8.7.
    • If you chose B: Milk is a drink, so it can seem light. But 100 g of milk holds far less than 100 g of groundnuts. Compare the numbers. Go to S3.8.7.
    • If you chose C: A potato is bulky, but bulk is not energy. Read the kJ numbers. Go to S3.8.7.

  2. A boiled egg has about 600 kJ in every 100 g. How much food energy is in 300 g of egg?

    Tap your answer.

    1. Yes, that is right. 300 g is 3 lots of 100 g. 3 × 600 = 1800 kJ.

    2. Not this one. You multiplied the grams by the kJ. First find how many lots of 100 g there are, then multiply. Go to S3.8.7. Try another.

    3. Not this one. You added. The kJ are for each 100 g, so multiply by the number of lots of 100 g. Go to S3.8.7. Try another.

    4. Not this one. That is only for the first 100 g. There are more lots of 100 g in this amount. Go to S3.8.7. Try another.

    See the answer

    1800 kJ

    300 g is 3 lots of 100 g. 3 × 600 = 1800 kJ.

    • If you chose B: You multiplied the grams by the kJ. First find how many lots of 100 g there are, then multiply. Go to S3.8.7.
    • If you chose C: You added. The kJ are for each 100 g, so multiply by the number of lots of 100 g. Go to S3.8.7.
    • If you chose D: That is only for the first 100 g. There are more lots of 100 g in this amount. Go to S3.8.7.

  3. Why do people who run or dig all day need more food energy?

    Tap your answer.

    1. Not this one. Food does not change. The body simply moves more energy out when it is active. Go to S3.8.7. Try another.

    2. Yes, that is right. Movement moves energy out of the chemical stores in the body. Food puts energy back in.

    3. Not this one. People differ. More movement takes more energy out of the body, so active people need more. Go to S3.8.7. Try another.

    See the answer

    Their bodies move more energy out, so they need more food to put it back.

    Movement moves energy out of the chemical stores in the body. Food puts energy back in.

    • If you chose A: Food does not change. The body simply moves more energy out when it is active. Go to S3.8.7.
    • If you chose C: People differ. More movement takes more energy out of the body, so active people need more. Go to S3.8.7.

  4. Digging moves about 1200 kJ out of the body each hour. How much energy does 4 hours of digging move?

    Tap your answer.

    1. Not this one. You divided. Each hour moves the same amount, so multiply the rate by the hours. Go to S3.8.7. Try another.

    2. Not this one. You added kJ and hours, which cannot be added. Multiply the kJ in one hour by the number of hours. Go to S3.8.7. Try another.

    3. Yes, that is right. 4 hours × 1200 kJ = 4800 kJ.

    See the answer

    4800 kJ

    4 hours × 1200 kJ = 4800 kJ.

    • If you chose A: You divided. Each hour moves the same amount, so multiply the rate by the hours. Go to S3.8.7.
    • If you chose B: You added kJ and hours, which cannot be added. Multiply the kJ in one hour by the number of hours. Go to S3.8.7.

  5. In what unit is the energy in food counted?

    Tap your answer.

    1. Not this one. Kilograms measure mass, which is how much stuff there is. Energy is counted in joules or kilojoules. Go to S3.8.7. Try another.

    2. Not this one. Degrees Celsius measure temperature, not energy. A cold food can hold a lot of energy. Go to S3.8.7. Try another.

    3. Not this one. Newtons measure a force. Energy is measured in joules. Go to S3.8.7. Try another.

    4. Yes, that is right. Food energy is counted in kilojoules. 1 kJ is 1000 J.

    See the answer

    Kilojoules, kJ

    Food energy is counted in kilojoules. 1 kJ is 1000 J.

    • If you chose A: Kilograms measure mass, which is how much stuff there is. Energy is counted in joules or kilojoules. Go to S3.8.7.
    • If you chose B: Degrees Celsius measure temperature, not energy. A cold food can hold a lot of energy. Go to S3.8.7.
    • If you chose C: Newtons measure a force. Energy is measured in joules. Go to S3.8.7.

  6. From an earlier chapter

    Review: a body takes in 200 J from food and moves with 40 J. What is the efficiency?

    Tap your answer.

    1. Yes, that is right. Efficiency = 40 ÷ 200 × 100 = 20%.

    2. Not this one. That is the wasted share. Efficiency uses the useful energy divided by the energy in. Go to S3.8.3. Try another.

    3. Not this one. You divided the energy in by the useful energy, upside down. Divide useful by in, then multiply by 100. Go to S3.8.3. Try another.

    See the answer

    20%

    Efficiency = 40 ÷ 200 × 100 = 20%.

    • If you chose B: That is the wasted share. Efficiency uses the useful energy divided by the energy in. Go to S3.8.3.
    • If you chose C: You divided the energy in by the useful energy, upside down. Divide useful by in, then multiply by 100. Go to S3.8.3.

Keep it

Copy this into your exercise book.

Food is a chemical store, counted in kilojoules. Foods hold different amounts. Active people move more energy out of their bodies, so they need more food energy.

Draw: Three bars: resting, walking, running. Make each one longer. Write kJ beside each bar.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (711 KB). It opens on any phone.