Grade by Grade
C. Matter and Energy · Forces Causing Movement

Gravity and Buoyancy: Pulling Down, Pushing Up

C2.1 A1.2About 35 minutes
Learning goals
  • I can explain that gravity is a non-contact force that pulls things toward the Earth.
  • I can explain that buoyancy is a contact force from water that pushes up on things.
  • I can explain why some things float and some things sink, using the words gravity and buoyancy.
  • I can do a fair test by changing only one thing.

Words to know

gravity
A non-contact force that pulls everything toward the Earth.
weight
How hard gravity pulls on something. Heavier things have more weight.
buoyancy
A contact force from water (or another liquid) that pushes up on things in it.
float
To stay on top of the water because the water pushes up as hard as gravity pulls down.
sink
To go down in water because gravity pulls down harder than the water can push up.
air resistance
A contact force from air that pushes against things moving through it and slows them down.
fair test
An experiment where you change only one thing and keep everything else the same.

What goes up must come down

Toss a ball up. It always comes back down. Jump as high as you can. You always land. Let go of a mitten, a spoon or a snowball, and down they go.

What pulls them? The force called gravity.

Gravity

  • Gravity is a non-contact force. It pulls without touching.
  • The Earth's gravity pulls everything toward the Earth. That is the direction we call down.
  • Gravity is why rain and snow fall, why rivers flow downhill, and why you stay on the ground instead of floating away.
  • Weight is how hard gravity pulls on something. A backpack full of books has more weight than an empty one, so it feels heavier.

The Moon has gravity too, but it is much weaker than Earth's. That is why astronauts on the Moon could jump very high and land slowly.

Why does a sheet of paper fall slowly?

Drop a flat sheet of paper and a crumpled paper ball at the same time. The ball lands first! Gravity pulls on both, but the air pushes up against the flat sheet. This push from the air is called air resistance. A flat shape catches more air, so it falls more slowly. That is how a parachute works, and how a maple key spins slowly to the ground.

Buoyancy: water pushes up

Remember from Grade 2: some things float and some things sink.

Now you can explain why. When something is in water, two forces act on it:

  • Gravity pulls it down.
  • The water pushes it up. This push is called buoyancy. It is a contact force: the water has to touch the object.

If the water can push up as hard as gravity pulls down, the object floats. If gravity pulls down harder than the water can push up, the object sinks.

You can feel buoyancy in a swimming pool: it is much easier to lift a friend in the water than on land.

A glass tank of water where an apple and a wooden block float at the top while a rock and a metal spoon rest on the bottom.
The apple and the wood float because the water pushes up on them as hard as gravity pulls down. The rock and the spoon sink because gravity wins. AI-generated image
gravity pulls downbuoyancy pushes upwater
A boat floats. Gravity pulls it down. The water pushes it up (buoyancy). The two forces are the same size, so the boat stays on top of the water.

Even things that sink feel buoyancy

A rock sinks, but the water still pushes up on it a little. That is why a rock feels lighter when you lift it under water.

Worked example

Let's read an experiment

Maya wanted to know: Does water push up on a can of soup? She put a can in a plastic bag and hung it from a rubber band. She measured how long the stretched rubber band was, first in the air and then with the can in a bowl of water. Here are her results.

  1. What changed? Only one thing: where the can was (in air or in water). Everything else stayed the same: the same can, the same rubber band, the same ruler. So it was a fair test.
  2. In air, the rubber band stretched to 14 cm. Gravity pulled the can down and stretched the band.
  3. In water, the band stretched to only 11 cm. That is 3 cm shorter.
  4. The band stretched less in water. Something must have been pushing the can up: the water's buoyancy.

Yes. Water pushes up on the can (buoyancy), so the rubber band does not stretch as far. The can still sinks, because gravity pulls harder than the water pushes up.

in air: 14 cmin water: 11 cm
Maya's experiment. The rubber band stretches less when the can is in the water.
Just for fun

Why did the boat always feel happy?

Because buoyancy kept lifting its spirits!

Hands-on activity

Feel the forces

You need: an empty plastic bottle with its cap on, a sink or bucket of water, a rubber band, a small plastic bag, a can of soup, a ruler, a sheet of paper and a crumpled ball of paper.

Part 1: Gravity and air. Stand on the floor (not a chair). Hold the flat sheet and the paper ball at the same height. Predict which will land first. Drop them together. Then crumple the sheet too and drop both again. What changed?

Part 2: Feel buoyancy. Push the empty capped bottle down into the water. Can you feel the water pushing back up? Let go. What happens?

Part 3: Maya's test. Put the can in the bag and hang the bag from the rubber band. A grown-up holds the end of the band. Measure how long the band is. Then lower the can into the bucket until it is under the water, and measure again. Record your numbers in a chart.

What to look for: the flat sheet falls more slowly than the ball, but two balls land together; the bottle pushes up hard and pops up when you let go; the rubber band is shorter when the can is in the water.

Safety: a grown-up helps with the water. Wipe up spills right away so nobody slips.

Your turn

1

Which force pulls a dropped spoon to the floor?

2

Is buoyancy a contact force or a non-contact force?

3

A beach ball floats in a lake. Name the two forces acting on it and which way each one acts.

4

A stone sinks to the bottom of a pond. Which force is stronger on the stone: gravity or buoyancy?

5

What is weight?

6

Why does a flat sheet of paper fall more slowly than a crumpled paper ball?

7

In a fair test, how many things do you change?

More practice: warm-up

1

Which way does gravity pull?

2

Is gravity a contact force or a non-contact force?

3

What pushes up on a swimmer in the water?

4

Does a rubber duck float or sink?

5

Why do rivers flow downhill?

More practice: on your own

1

Why is it easier to lift your friend in a swimming pool than on the grass?

2

A full backpack and an empty backpack: which one has more weight? Why?

3

Leo drops a feather and a marble. The marble lands first. Is gravity weaker on the feather? Explain.

4

In Maya's experiment, the band was 14 cm in air and 11 cm in water. How much shorter was it in water? What does that tell you?

5

Ana wants to test if a ball falls faster from higher up. She drops it from a chair and from a table, but she uses a tennis ball one time and a ping-pong ball the other time. Is this a fair test? Why?

Challenge

1

An empty plastic bottle floats. Then you fill it with sand and put the cap on. Now it sinks. Use the words gravity and buoyancy to explain why.

2

A child on the Moon throws a ball up. Will it come down? Explain.

3

Design a fair test to find out whether a big or a small parachute (made from a plastic bag) falls more slowly. What will you change? What will you keep the same? What will you measure?

Check your understanding

Quiz

1.

Which force pulls things down toward the Earth?

2.

Buoyancy is a force from water that pushes...

3.

A toy boat floats. What is true?

4.

A metal spoon sinks. Why?

5.

What does weight tell us?

6.

In a fair test you should...

For parents and teachers

Big idea: Gravity is a non-contact force that pulls everything toward Earth; buoyancy is a contact force from a liquid that pushes up. Whether something floats or sinks depends on which is stronger. This builds on Grade 2 float/sink work and on Ontario Grade 3 expectation C2.1 (contact and non-contact forces). Planning a fair test (change one thing, keep the rest the same, measure) is the scientific experimentation process in Strand A (A1.2).

How to help: At bath time or at the pool, let your child push floating toys under the water and feel the push back. Ask "Which force is pulling? Which is pushing?"

Common difficulties:

  • Thinking heavier things always fall faster. Without air in the way, a heavy and a light object dropped together land together. A feather or a flat sheet is slowed by air resistance, not by weaker gravity.
  • Thinking there is no buoyancy on things that sink. The water still pushes up; it just isn't enough.
  • Changing two things in a test. Ask: "What is the one thing you are changing?"

Safety: always supervise water play, and wipe spills to prevent slips. Drop things from standing height, not from chairs or stairs.