Chapter 2 of 8

All 8 chapters
  1. 1Forces are pushes and pulls, readunderstood
  2. 2Measuring forces, readunderstood
  3. 3Friction, readunderstood
  4. 4More friction or less, readunderstood
  5. 5Gravity and falling, readunderstood
  6. 6Air and water resistance, readunderstood
  7. 7Floating and sinking, readunderstood
  8. 8Shape and floating, readunderstood

S2.5.2 Chapter 2

Measuring forces

How can I tell how big a pull is?

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

  • Make a force meter with a rubber band and a ruler
  • Explain that forces are measured in newtons

A pull can be small or big. Your hand can tell you a little. A ruler can tell you more.

A rubber band gets longer when you pull it. We call this a stretch. A bigger pull gives a bigger stretch.

So the stretch of a band shows how big the pull is. A band and a ruler make a force meter. A force meter measures a pull.

Draw it first

Draw a force meter

  1. A pencil rests across the top, labelled pencil. A ruler stands at the left with cm marks from 0 to 7 and the label cm. A rubber band hangs from the pencil, labelled band, with a cup under it, labelled cup. A dashed line runs from the cup to the ruler at 2. A second band hangs from the pencil with a cup that holds three stones, labelled stones. Its dashed line runs to the ruler at 5. Below is the label more stones, more stretch.pencil01234567cmbandcup

    1 Draw a pencil, a ruler and a band with an empty cup. Mark 2 cm.

  2. A pencil rests across the top, labelled pencil. A ruler stands at the left with cm marks from 0 to 7 and the label cm. A rubber band hangs from the pencil, labelled band, with a cup under it, labelled cup. A dashed line runs from the cup to the ruler at 2. A second band hangs from the pencil with a cup that holds three stones, labelled stones. Its dashed line runs to the ruler at 5. Below is the label more stones, more stretch.pencil01234567cmbandcupstones

    2 Draw a second band. Put three stones in its cup. Mark 5 cm.

  3. A pencil rests across the top, labelled pencil. A ruler stands at the left with cm marks from 0 to 7 and the label cm. A rubber band hangs from the pencil, labelled band, with a cup under it, labelled cup. A dashed line runs from the cup to the ruler at 2. A second band hangs from the pencil with a cup that holds three stones, labelled stones. Its dashed line runs to the ruler at 5. Below is the label more stones, more stretch.pencil01234567cmbandcupstonesmore stones, more stretch

    3 Write more stones, more stretch.

Put in the stones one at a time. Read the band each time. Write it in a table.

See it three ways

As a picture

A bar chart called length in cm. Under the bars are the numbers 0, 1, 2 and 3, for the stones in the cup. The bars grow taller from left to right: 2 cm, 3 cm, 4 cm and 5 cm. Labels: length in cm, 0, 1, 2, 3.01234560123length in cm

In words

These numbers come from one test. Each stone pulls the band a little more. The band gets longer each time. More pull, more stretch. Your numbers will differ from ours.

In symbols

0 stones: 2 cm. 3 stones: 5 cm. Three stones stretch the band 3 cm more.

The unit of force is the newton. We write N. It is named after Isaac Newton.

The Earth pulls on a small apple with about 1 newton. That apple is about 100 grams.

People often say a sack weighs 5 kilograms. That is fine in daily life. In science, kilograms tell how much stuff a thing has. Newtons tell how strong a pull is.

Hang a small apple in your cup. Mark how much the band grows. That is about 1 newton. Two apples make about 2 newtons.

A real force meter has a spring in a tube. A pointer moves along a scale. The scale is marked in newtons.

Three force meters side by side. Each has a scale with the marks 0, 1, 2, 3 and 4, and the label N for newtons. The first holds one apple, labelled apple, and its pointer stands at 1. The second holds two apples, labelled 2 apples, and its pointer stands at 2. The third holds three apples, labelled 3 apples, and its pointer stands at 3.01234Napple01234N2 apples01234N3 apples
Each small apple pulls with about 1 N. More apples, more newtons.

Try it

Make a force meter

You need: a rubber band, a pencil, two piles of books, a small cup or a small bag with string, a ruler, about six stones of the same size, or beans, paper and a pencil.

Be safe: Keep your face away from the band. Stop if the band looks very thin.

  1. Rest the pencil across two tall piles of books. Hang the band from it.
  2. Hang the cup from the band. Hold the ruler upright. Put its 0 at the top of the band.
  3. Read how long the band is. Write the number.
  4. Put in one stone. Read the band again. Write it.
  5. Add stones one at a time. Read each time. Stop at four.

What you will see: The band gets longer with every stone. A bigger pull gives a bigger stretch. Your numbers will not be the same as ours. That is fine.

If you cannot try it, this is what happens.
A table with two columns. The head row says Stones and Band. Row one: 0, 2 cm. Row two: 1, 3 cm. Row three: 2, 4 cm. Row four: 3, 5 cm.StonesBand02 cm13 cm24 cm35 cm

A wrong idea many people have

You can only feel how big a force is. You cannot measure it.

Why it feels right: A force is not a thing you can see or hold. It is only a feeling in your hand.

The test that breaks it: Hang three stones on your force meter. Read the band. Take them off. Hang the same three stones again. You get the same reading. One stone gives a smaller reading.

What is true: A force can be measured. The stretch of a band gives a number. The same pull gives the same number.

Key words

stretch
to get longer when you pull.
force meter
a tool that measures a pull.
newton
the unit for force. We write N.

Back to your guess

The answer is B: A rubber band stretches more for a bigger pull.

A rubber band stretches more for a bigger pull. We can read the stretch on a ruler. That measures the pull.

Check yourself

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

  1. Look at the table. How long is the band with 2 stones?

    A table with two columns. The head row says Stones and Band. Row one: 0, 2 cm. Row two: 1, 3 cm. Row three: 2, 4 cm. Row four: 3, 5 cm.StonesBand02 cm13 cm24 cm35 cm
    Tap your answer.

    1. Not this one. You read the number of stones, or the empty band. Find the row for 2 stones. Try another.

    2. Not this one. That is the length with 3 stones. Find the row for 2 stones. Try another.

    3. Yes, that is right. The row for 2 stones says 4 cm.

    4. Not this one. That is the length with 1 stone. The first row is 0 stones. So 2 stones is row three. Try another.

    See the answer

    4 cm

    The row for 2 stones says 4 cm.

    • If you chose A: You read the number of stones, or the empty band. Find the row for 2 stones.
    • If you chose B: That is the length with 3 stones. Find the row for 2 stones.
    • If you chose D: That is the length with 1 stone. The first row is 0 stones. So 2 stones is row three.

  2. The band is 2 cm long with an empty cup. It is 5 cm long with three stones. How much longer is it with the stones?

    Tap your answer.

    1. Not this one. That is the whole length with the stones. Take away the 2 cm it had before. Try another.

    2. Not this one. You added the two lengths. Take the short one away from the long one. Try another.

    3. Not this one. That is how long the band was at the start. Find how much it grew. Try another.

    4. Yes, that is right. The band grew from 2 cm to 5 cm. 5 − 2 = 3 cm.

    See the answer

    3 cm

    The band grew from 2 cm to 5 cm. 5 − 2 = 3 cm.

    • If you chose A: That is the whole length with the stones. Take away the 2 cm it had before.
    • If you chose B: You added the two lengths. Take the short one away from the long one.
    • If you chose C: That is how long the band was at the start. Find how much it grew.

  3. In what unit do scientists measure force?

    Tap your answer.

    1. Yes, that is right. Force is measured in newtons. We write N.

    2. Not this one. Kilograms tell how much stuff a thing has. A force is a push or a pull. Try another.

    3. Not this one. Centimetres tell how long the band is. That shows the pull, but it is not the unit of force. Try another.

    See the answer

    Newtons

    Force is measured in newtons. We write N.

    • If you chose B: Kilograms tell how much stuff a thing has. A force is a push or a pull.
    • If you chose C: Centimetres tell how long the band is. That shows the pull, but it is not the unit of force.

  4. Ravi hangs a pebble in the cup. The band is 3 cm long. A bigger stone makes it 5 cm. Which pull is bigger?

    Tap your answer.

    1. Not this one. The band stretches more for a bigger pull. The longer band shows the bigger pull. Try another.

    2. Not this one. The lengths are different, so the pulls are different. Try another.

    3. Yes, that is right. The band is longer with the bigger stone. More stretch means more pull.

    See the answer

    The bigger stone pulls more.

    The band is longer with the bigger stone. More stretch means more pull.

    • If you chose A: The band stretches more for a bigger pull. The longer band shows the bigger pull.
    • If you chose B: The lengths are different, so the pulls are different.

  5. From an earlier chapter

    A stone hangs still from a band. The Earth pulls the stone down. What does the band do?

    Tap your answer.

    1. Yes, that is right. The two forces are balanced, so the stone stays still.

    2. Not this one. A still thing can have forces on it. They are balanced. Go to S2.5.1. Try another.

    3. Not this one. A stretched band pulls back towards where it started. That is up. Go to S2.5.1. Try another.

    See the answer

    It pulls the stone up as hard as the Earth pulls down.

    The two forces are balanced, so the stone stays still.

    • If you chose B: A still thing can have forces on it. They are balanced. Go to S2.5.1.
    • If you chose C: A stretched band pulls back towards where it started. That is up. Go to S2.5.1.

  6. Draw a force meter with two stones in the cup. Write the reading from the table.

    See the answer

    A band hangs from a pencil. Two stones are in the cup. The reading is 4 cm.

    The table says 4 cm for 2 stones.

  7. Priya hangs a stone from a force meter. Look at the picture. What does the meter read?

    A force meter hangs by a ring. Its scale has the marks 0, 1, 2, 3, 4 and 5, with the label N for newtons. A pointer stands at the mark 3. A stone, labelled stone, hangs in a bag from the hook.012345Nstone
    Tap your answer.

    1. Not this one. This meter measures a pull. Its scale is in newtons, N. Kilograms tell how much stuff a thing has. Try another.

    2. Yes, that is right. The pointer stands at the mark 3. The scale is in newtons. The stone pulls with 3 N.

    3. Not this one. That is the biggest number on the scale. Read the number at the pointer, not at the end. Try another.

    4. Not this one. The pointer stands at 0 N when nothing hangs. The stone pulls it down to a bigger number. Try another.

    See the answer

    3 N

    The pointer stands at the mark 3. The scale is in newtons. The stone pulls with 3 N.

    • If you chose A: This meter measures a pull. Its scale is in newtons, N. Kilograms tell how much stuff a thing has.
    • If you chose C: That is the biggest number on the scale. Read the number at the pointer, not at the end.
    • If you chose D: The pointer stands at 0 N when nothing hangs. The stone pulls it down to a bigger number.

Keep it

Copy this into your exercise book.

A rubber band stretches more for a bigger pull. A band and a ruler make a force meter. Force is measured in newtons, written N. A small apple weighs about 1 newton.

Draw: A band hanging from a pencil. A cup of stones. A ruler beside it.

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 (691 KB). It opens on any phone.