Chapter 4 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

S2.5.4 Chapter 4

More friction or less

How can we get less friction, or more?

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.

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

By the end you can

  • Give ways to reduce friction, such as oil, water or wheels
  • Explain why shoe soles and tyres have grooves

Sometimes we want less friction. Sometimes we want more. We can change it.

Draw it first

Draw three ways to slide a box

  1. Three rows. In each row a box sits on the ground with an arrow pointing left, labelled friction. In the row labelled sliding, the box rests on the ground and the arrow is long. In the row labelled oil, a thin layer of oil lies under the box and the arrow is shorter. In the row labelled rollers, the box rests on three round pencils and the arrow is the shortest.frictionsliding

    1 Draw a box on the ground. Draw a long friction arrow pointing left.

  2. Three rows. In each row a box sits on the ground with an arrow pointing left, labelled friction. In the row labelled sliding, the box rests on the ground and the arrow is long. In the row labelled oil, a thin layer of oil lies under the box and the arrow is shorter. In the row labelled rollers, the box rests on three round pencils and the arrow is the shortest.frictionslidingfrictionoil

    2 Draw oil under a box. Draw a shorter friction arrow.

  3. Three rows. In each row a box sits on the ground with an arrow pointing left, labelled friction. In the row labelled sliding, the box rests on the ground and the arrow is long. In the row labelled oil, a thin layer of oil lies under the box and the arrow is shorter. In the row labelled rollers, the box rests on three round pencils and the arrow is the shortest.frictionslidingfrictionoilfrictionrollers

    3 Draw a box on round pencils. Draw the shortest friction arrow.

Oil gives less friction. A drop of oil between moving parts lets them slide. A bicycle chain needs oil.

Water can give less friction too. A wet, smooth floor is slippery.

A roller is a round thing that rolls under a load. Wheels and rollers give less friction. Rolling rubs much less than sliding. A heavy box on rollers needs a smaller push.

Try it

Slide it, then roll it

You need: a heavy book, string, the band from your force meter, a table, three round pencils, a ruler.

Be safe: Keep your face away from the band. Do not let the book fall on your feet.

  1. Tie the string round the book. Tie the band to the string.
  2. Pull the band slowly along the table. Keep it steady. Read the length of the band as the book slides.
  3. Put three round pencils under the book, side by side. Pull again. Move each pencil to the front once the book has passed it.
  4. Read the band again. Which pull was bigger?

What you will see: The band is shorter when the book rolls on the pencils. A shorter band means a smaller pull. Rolling gives less friction than sliding. Our band went from 6 cm to 3 cm. Yours may differ.

If you cannot try it, this is what happens.
Two rows. In the row labelled sliding, a box labelled book lies on the ground. A line from the book shows the band, which is 6 cm long, labelled 6 cm. In the row labelled on pencils, a box labelled book rests on three round pencils. Its band is shorter, labelled 3 cm.book6 cmslidingbook3 cmon pencils

We also need more friction. A rubber mat stops a cup sliding. Chalk on the hands of a climber helps them hold. Sand on a slippery path helps your shoes grip.

Three rows. In each row a box sits on the ground with an arrow pointing left, labelled friction. In the row labelled rubber mat, the box rests on a hatched mat and the arrow is the longest. In the row labelled dry floor, the box rests on the ground and the arrow is shorter. In the row labelled wet floor, a thin layer of water, labelled water, lies under the box with a few drops beside it, and the arrow is the shortest.frictionrubber matfrictiondry floorfrictionwaterwet floor
A mat gives a lot of friction. A wet floor gives very little.

To grip is to hold on without slipping. Shoes grip best when the rubber touches the ground.

Wet or muddy ground is slippery. Water and mud get between the sole and the ground. The shoe slides.

See it three ways

As a picture

A close up of two soles on wet ground. The top sole is smooth, labelled smooth sole. A layer of water, labelled water, lies between it and the dark ground. The lower sole, labelled grooved sole, has gaps between blocks. Water, labelled water, sits in the gaps. The blocks touch the ground.smooth solewatergrooved solewater

In words

A groove is a narrow gap cut in the sole. Water and mud go into the grooves. The rubber touches the ground, and the shoe grips.

Tyres have grooves for the same reason. On a wet road, grooves move water from under the tyre. On a dry road, a smooth tyre can grip well.

A wrong idea many people have

Friction is always bad. We should get rid of all of it.

Why it feels right: Friction slows things down and makes them warm. It seems like a waste.

The test that breaks it: Try to turn a tight bottle top with dry fingers. It turns. Put a drop of oil or soapy water on your fingers. Try again. Your fingers slip. With less friction you cannot hold on. Wash your hands afterwards.

What is true: We need friction to hold, walk and stop. We want less in some places and more in others.

Key words

roller
a round thing that rolls under a load.
grip
to hold on without slipping.
groove
a narrow gap cut in a sole or a tyre.

Back to your guess

The answer is C: Oil, water and wheels give less friction.

Oil, water and wheels give less friction. Rough rubber and sand give more grip. Grooves help on wet or muddy ground.

Check yourself

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

  1. Which gives less friction under a heavy box?

    Tap your answer.

    1. Not this one. A rough mat catches the box. It gives more friction. Try another.

    2. Yes, that is right. Rolling rubs much less than sliding. Rollers give less friction.

    3. Not this one. Sand makes a surface rougher. It gives more friction. Try another.

    See the answer

    Round rollers

    Rolling rubs much less than sliding. Rollers give less friction.

    • If you chose A: A rough mat catches the box. It gives more friction.
    • If you chose C: Sand makes a surface rougher. It gives more friction.

  2. Why does a bicycle chain get a drop of oil?

    Tap your answer.

    1. Not this one. Oil does the opposite. It makes surfaces slide, so there is less friction. Try another.

    2. Not this one. Friction makes heat. Oil gives less friction, so less heat. Go to S2.5.3. Try another.

    3. Yes, that is right. The oil lets the parts slide past each other. There is less friction.

    See the answer

    The oil gives less friction between the moving parts.

    The oil lets the parts slide past each other. There is less friction.

    • If you chose A: Oil does the opposite. It makes surfaces slide, so there is less friction.
    • If you chose B: Friction makes heat. Oil gives less friction, so less heat. Go to S2.5.3.

  3. Why do boots for mud have grooves?

    Tap your answer.

    1. Yes, that is right. Mud and water move into the grooves. The rubber touches the ground and grips.

    2. Not this one. Grooves are gaps. They give mud and water a place to go. Try another.

    3. Not this one. The grooves are there for grip, not for weight. Try another.

    See the answer

    Mud goes into the grooves, so the rubber can grip.

    Mud and water move into the grooves. The rubber touches the ground and grips.

    • If you chose B: Grooves are gaps. They give mud and water a place to go.
    • If you chose C: The grooves are there for grip, not for weight.

  4. Look at the picture. Which row has the least friction?

    Three rows. In each row a box sits on the ground with an arrow pointing left, labelled friction. In the row labelled sliding, the box rests on the ground and the arrow is long. In the row labelled oil, a thin layer of oil lies under the box and the arrow is shorter. In the row labelled rollers, the box rests on three round pencils and the arrow is the shortest.frictionslidingfrictionoilfrictionrollers
    Tap your answer.

    1. Not this one. The friction arrow in this row is the longest. It has the most friction. Try another.

    2. Not this one. The oil row has a short arrow. The rollers row has a shorter one. Try another.

    3. Not this one. The arrows differ in length, so the friction differs. Try another.

    4. Yes, that is right. The friction arrow is the shortest in the rollers row.

    See the answer

    rollers

    The friction arrow is the shortest in the rollers row.

    • If you chose A: The friction arrow in this row is the longest. It has the most friction.
    • If you chose B: The oil row has a short arrow. The rollers row has a shorter one.
    • If you chose C: The arrows differ in length, so the friction differs.

  5. From an earlier chapter

    A box slides to the right and stops. Which way does the friction arrow point?

    Tap your answer.

    1. Not this one. Friction acts against the slide. It does not help the box along. Go to S2.5.3. Try another.

    2. Yes, that is right. Friction pushes against the way a thing slides. The box slides right, so friction points left.

    3. Not this one. Friction acts along the surface, against the way the box slides. Go to S2.5.3. Try another.

    See the answer

    Left, against the slide

    Friction pushes against the way a thing slides. The box slides right, so friction points left.

    • If you chose A: Friction acts against the slide. It does not help the box along. Go to S2.5.3.
    • If you chose C: Friction acts along the surface, against the way the box slides. Go to S2.5.3.

  6. Write two ways to get less friction. Write two ways to get more.

    See the answer

    Less: oil, or wheels and rollers. More: grooved soles, or sand on a slippery path.

    Oil and rollers help things slide. Grooves and sand help things grip.

  7. Why is a wet, smooth floor slippery?

    Tap your answer.

    1. Not this one. Cold is not the reason. The water between shoe and floor gives less friction. Try another.

    2. Not this one. A film of water makes a floor smoother, not rougher. Try another.

    3. Not this one. It is the other way round. Water gets between shoe and floor. The shoe slides. Try another.

    4. Yes, that is right. A film of water lets the shoe slide. With less friction your foot slips.

    See the answer

    Water between the shoe and the floor gives less friction.

    A film of water lets the shoe slide. With less friction your foot slips.

    • If you chose A: Cold is not the reason. The water between shoe and floor gives less friction.
    • If you chose B: A film of water makes a floor smoother, not rougher.
    • If you chose C: It is the other way round. Water gets between shoe and floor. The shoe slides.

  8. A climber puts chalk on her hands. Why?

    Tap your answer.

    1. Yes, that is right. Chalk gives more friction between the hands and the rock. The hands do not slip.

    2. Not this one. Chalk is not oil. It helps the hands hold on without slipping. Try another.

    3. Not this one. Chalk dries the hands. Sweat is slippery. Dry hands grip the rock better. Try another.

    See the answer

    Chalk gives the hands more grip.

    Chalk gives more friction between the hands and the rock. The hands do not slip.

    • If you chose B: Chalk is not oil. It helps the hands hold on without slipping.
    • If you chose C: Chalk dries the hands. Sweat is slippery. Dry hands grip the rock better.

  9. Look at the picture. Leila walks on a rubber mat and then on a wet floor. Where is she more likely to slip?

    Two rows. In each row a box sits on the ground with an arrow pointing left, labelled friction. In the row labelled rubber mat, the box rests on a hatched mat and the arrow is long. In the row labelled wet floor, a thin layer of water, labelled water, lies under the box with a few drops beside it, and the arrow is short.frictionrubber matfrictionwaterwet floor
    Tap your answer.

    1. Not this one. The friction arrow is long on the mat. A mat gives a lot of friction, so the shoe grips. Try another.

    2. Yes, that is right. The friction arrow is the shortest on the wet floor. With little friction a shoe slips.

    3. Not this one. The arrows differ in length, so the friction differs. Try another.

    See the answer

    On the wet floor

    The friction arrow is the shortest on the wet floor. With little friction a shoe slips.

    • If you chose A: The friction arrow is long on the mat. A mat gives a lot of friction, so the shoe grips.
    • If you chose C: The arrows differ in length, so the friction differs.

Keep it

Copy this into your exercise book.

Oil, water and wheels give less friction. Rough rubber and sand give more grip. On wet or muddy ground, grooves help shoes and tyres grip. Water and mud go into the grooves. The rubber touches the ground.

Draw: A sole with grooves on wet ground. Water in the grooves.

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