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Primary Science · Year 5Chapter 9 · Gravity, friction and resistance
Year 5 Science · Gravity, friction and resistance · Lesson 9.5

Year 5 Science: Comparing gravity, resistance and friction

Part of the Year 5 Science gravity, friction and resistance chapter. Chapter overview · whole chapter on one page

9.5

Comparing gravity, resistance and friction

Year 5
F1 and F2: compare the forces that act on falling and moving objects, and decide which force is acting
Story. Priya drops a sheet of paper and a small stone from the same height. The stone hits the floor first. “Gravity pulls them both,” she says. “So why does the paper fall more slowly?” Mr Patel says: “Air resistance pushes up against the paper.”

Several forces can act on a falling or moving object at the same time:

ForceWhat it doesDirection it acts
gravitypulls objects towards the Earthdownwards, towards the centre of the Earth
air resistancepushes against moving things in airagainst the motion
water resistancepushes against moving things in wateragainst the motion
frictionacts between surfaces that rubagainst the motion

Gravity pulls every object towards the Earth. Air resistance, water resistance and friction all push against the motion. This is why a falling object can fall more slowly than gravity alone would make it fall.

Key ideas

  • Gravity pulls objects towards the Earth, downwards.
  • Air resistance, water resistance and friction act against the motion.
  • Gravity acts at a distance, but friction needs surfaces that touch and rub.
  • Several forces can act together on the same object.
Worked example 9.5.1
Which force pulls a falling object towards the Earth? Write the word.

Method. Read the table.

  1. Read the first row. The first row says gravity pulls objects towards the Earth.
    gravity
  2. Name the force. So the force is gravity.
    gravity
  3. Write the answer.
    gravity

Check. Gravity acts on every object. ✓

Worked example 9.5.2
Which force slows a paper sheet falling through air? Write the words.

Method. Think about the paper in air.

  1. Think about the air. The paper moves through air, and air pushes against it.
    pushes against it
  2. Name the force. The force is air resistance.
    air resistance
  3. Write the answer.
    air resistance

Check. The flat paper has more air resistance than a crumpled ball. ✓

Worked example 9.5.3
Which force slows a box sliding across the floor? Write the word.

Method. Think about the box and the floor.

  1. Think about the surfaces. The box and the floor rub together.
    rub together
  2. Name the force. The force that slows the box is friction.
    friction
  3. Write the answer.
    friction

Check. Friction acts between the box and the floor. ✓

Watch out

Watch out: gravity is always there, but air resistance, water resistance and friction only act when something is moving through air or water, or when surfaces rub.

Try it (with your teacher or a grown-up). With your teacher, drop a sheet of paper flat and then crumpled into a ball, from the same height, onto a rug. Then drop a small stone, the same height. Which falls first? Which force makes the difference? Write one sentence about your result.

Exam tip

Grown-up tip. Ask: “Which force pulls everything down? Which force pushes against the paper?” Keep the drop low, on a soft surface.

Exercise 9E  Comparing gravity, resistance and friction

1
FWhich force pulls objects towards the Earth? Write the word.
2
FWhich force pushes against a falling object, through air? Write the words.
3
FWhich force slows a boat moving through water? Write the words.
4
FWhich force acts between two surfaces that rub? Write the word.
5
PSWhich force acts at a distance, so it does not need the objects to touch? Write the word.
6
PSWhich force needs two surfaces that rub: gravity, or friction? Write the word.
7
PSDoes a force from air act against a falling paper sheet? Write yes or no.
8
PSPriya says: “Air resistance pulls the paper to the ground.” Is Priya right? Write yes or no.
9
PSWorking scientifically. Compare forces. Which of these forces acts on a falling paper sheet, and against its motion: gravity, or air resistance? Write the word.
10
RExplain why a falling paper sheet takes longer to reach the floor than a stone dropped from the same height. Write one sentence.
11
RExplain why both objects in section 9.5 are pulled by gravity, even though they fall at different speeds. Write one sentence.

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