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Primary Science · Year 3Chapter 9 · Forces
Year 3 Science · Forces · Lesson 9.6

Year 3 Science: Air resistance and water resistance

Part of the Year 3 Science forces chapter. Chapter overview · whole chapter on one page

9.6

Air resistance and water resistance

Year 3
F: forces act on moving objects; air resistance and water resistance slow down moving things
Story. Priya drops two sheets of the same paper from the same height. One stays flat, and the other is crumpled into a tight ball. The flat sheet floats down slowly, and the ball drops quickly. “Why does the flat sheet fall more slowly?” she asks. Mr Patel says, “Air is pushing against it. That is air resistance.”

Air resistance is a force that acts on moving things in the air. It pushes against them and slows them down. A bigger flat surface catches more air, so it has more air resistance. A parachute is wide, so it catches a lot of air and falls slowly.

Water resistance is a force that acts on moving things in water. It pushes against them and slows them down. A flat plate moved sideways through water feels harder to move than the same plate moved edge-first.

Flat sheetCrumpled ballbig flat shape: more airresistance, falls slowlysmall shape: less airresistance, falls quickly
Two shapes falling. The flat sheet has four air arrows pushing up, so it has more air resistance. The crumpled ball has one air arrow.
water resistancemotiona flat plate moved sideways through water
A flat plate moves sideways through water. Water resistance pushes back against the motion.

The table shows the results of a fair test. Priya dropped each shape from the same height, three times, and timed how long it took to reach the floor.

TrialFlat sheet (seconds)Crumpled ball (seconds)
Trial 12.00.6
Trial 22.20.5
Trial 32.10.7
Average2.10.6

Key ideas

  • Air resistance is a force that slows down moving things in air.
  • Air resistance pushes against a falling object, in the opposite direction to its motion.
  • A bigger flat surface catches more air, so it has more air resistance.
  • Water resistance is a force that slows down moving things in water.
Worked example 9.6.1
Look at the table in section 9.6. Which shape falls more slowly, on average: the flat sheet or the crumpled ball? Write the shape.

Method. Compare the averages.

  1. Read the averages. The flat sheet averages 2.1 seconds, and the crumpled ball averages 0.6 seconds.
    2.1 and 0.6
  2. Choose the slower one. The longer time is for the flat sheet, so it falls more slowly.
    flat sheet
  3. Write the answer.
    flat sheet

Check. The flat sheet catches more air, so it has more air resistance. ✓

Watch out. The shape that takes longer to reach the floor falls more slowly.

Worked example 9.6.2
Look at the table in section 9.6. How many seconds longer did the flat sheet take than the crumpled ball, on average? Write the number.

Method. Subtract the averages.

  1. Read the averages. The flat sheet averages 2.1 seconds, and the crumpled ball averages 0.6 seconds.
    2.1 and 0.6
  2. Subtract. The difference is 2.1 minus 0.6, which is 1.5.
    2.1 - 0.6 = 1.5
  3. Write the answer.
    1.5

Check. Check: 0.6 + 1.5 = 2.1. ✓

Watch out. Take the smaller time away from the larger time.

Worked example 9.6.3
A parachute is wide and catches a lot of air. Does a parachute have more air resistance or less air resistance than a small tight ball? Write more or less.

Method. Think about the air that each one catches.

  1. Think about the parachute. A parachute is wide, so it catches a lot of air.
    catches a lot of air
  2. Compare. A small tight ball catches less air, so the parachute has more air resistance.
    more
  3. Write the answer.
    more

Check. More air caught means more air resistance. ✓

Watch out

Watch out: the crumpled ball is not heavier than the flat sheet. It is made of the same paper, so only its shape is different.

Try it at home or school (with a grown-up). With a grown-up, take two sheets of the same paper. Drop one flat, and crumple the other into a tight ball. Drop both from the same height, one at a time, onto a rug or a soft floor. Watch which one falls more slowly. Repeat three times for each shape, and find the averages. Stand on the floor, and never climb on a chair to drop them.

Exam tip

Grown-up tip. Ask: “Which one fell more slowly? Which one caught more air?” Let your child explain the air pushing up, in their own words.

Exercise 9F  Air resistance and water resistance

1
FWhat is the name of the force that slows down a moving object in air? Write the words.
2
FWhat is the name of the force that slows down a moving object in water? Write the words.
3
FFill in the gap: air resistance pushes ___ a falling object. Write the word.
4
FWhich falls more slowly: a flat sheet of paper or a crumpled ball of the same paper? Write the shape.
5
FLook at the table in section 9.6. What is the average time for the crumpled ball? Write the number of seconds.
6
FLook at the table in section 9.6. What is the time for the flat sheet in trial 2? Write the number of seconds.
7
FDoes a bigger flat surface catch more air or less air? Write more or less.
8
FTrue or false: water resistance is a force that acts on moving things in water. Write true or false.
9
PSMo says a crumpled ball of paper is heavier than the same paper when it is flat, so it falls faster. Is Mo right? Write no or yes.
10
PSWorking scientifically. Predict whether a flat plate or a thin edge of the same plate will feel harder to move through water. Write one prediction.
11
RExplain why a parachute falls more slowly than a small ball. Write one sentence.

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