C. Matter and Energy · Forces Causing Movement

Forces Change Motion and Shape

C2.3 A1.2 A2.1About 40 minutes

Learning goals

I can explain that a force can make an object start, stop, change direction, speed up, slow down or change shape.
I can explain that a bigger force makes a bigger change.
I can tell when forces are balanced or unbalanced.
I can measure the effect of forces of different sizes, and write and debug code that models a push.

Words to know

motion
Moving. An object in motion is changing its position.

speed
How fast something moves.

direction
The way something is moving or facing, like left, right, up or down.

magnitude
The size or strength of a force. A big force has a large magnitude.

balanced forces
Forces that are the same size but push or pull in opposite directions. They cancel out, so the motion does not change.

unbalanced forces
Forces that are not the same size. The object starts moving, stops, or changes speed or direction.

debug
To find and fix a mistake in code.

Remember from Grade 2

In Grade 2 you learned that a force can start something moving, stop it, or change its direction. In Grade 3 we look closely at more changes (speeding up, slowing down and changing shape), and we look at how big the force is.

What can a force do?

A force can make an object:

Start moving: you kick a soccer ball that was sitting still.
Stop: a goalie catches the puck.
Change direction: a ball bounces off a wall and comes back. The wall pushed it.
Speed up: you push harder on the pedals of your bike.
Slow down: you squeeze the brakes (friction!).
Change shape: you squeeze a snowball, step on an empty juice box, or stretch an elastic band.

A tennis ball hitting the strings of a tennis racket on an outdoor court. — The racket pushes on the ball and changes its direction and speed. What other changes could a force make to the ball? AI-generated image

Action	What changed?
A dog pulls its leash and the wagon starts rolling.	start
A catcher's mitt catches a baseball.	stop
A hockey player hits a moving puck toward the net.	direction (and speed)
A sledder pushes off the snow with her hands.	speed up
A cyclist squeezes the brakes.	slow down
You press your thumb into play dough.	shape

Forces cause changes

Bigger force, bigger change

The magnitude of a force is how big or how strong it is.

A gentle tap on a ball makes it roll a little way, slowly.
A strong kick makes it roll fast and far.
A gentle squeeze dents a sponge a little. A strong squeeze flattens it.

The bigger the force, the bigger the change.

The same ball. A small push makes it roll a short way. A big push makes it roll much farther.

Changing shape: does it spring back?

When a force changes an object's shape, two things can happen:

Some objects spring back when the force stops: a sponge, a rubber ball, an elastic band, a trampoline.
Some objects stay changed: play dough, a crushed pop can, a footprint in the snow.

Balanced and unbalanced forces

Often two or more forces act on the same object.

In tug-of-war, if both teams pull equally hard, the rope does not move. The forces are balanced.
If one team pulls harder, the rope moves toward that team. The forces are unbalanced.

Balanced forces → no change in motion. Unbalanced forces → a change (start, stop, speed up, slow down or change direction).

Tug-of-war. When both teams pull equally, the forces are balanced and the rope stays still. When one team pulls harder, the forces are unbalanced and the rope moves.

Worked example

Let's measure the size of a push

Noor put a crumpled paper ball on a table and blew it with a straw. She tried a soft puff, a medium puff and a hard puff, one big breath each time, and measured how far the ball rolled. She did each puff 3 times and used the middle result.

soft puff: 12 cm
medium puff: 30 cm
hard puff: 55 cm

What changed? Only the size of the push (soft, medium, hard). Same ball, same straw, same table. It was a fair test.
Put the results in order: 12 cm, 30 cm, 55 cm. As the push got bigger, the distance got bigger.
Compare the hard and soft puffs: 55 − 12 = 43 cm farther.
Noor can show her results in a bar graph, with one bar for each puff. The hard-puff bar is the tallest.

A bigger force made the ball roll farther. The hard puff sent it 43 cm farther than the soft puff.

Coding a push

Coders can model (copy) how forces work. Here is code for a game where you choose how hard to push a toy car. The car moves more steps for a bigger push, and it bounces back when it hits a wall.

when the green flag is clicked

ask "How strong is your push? 1, 2 or 3"

if answer = 1 then move 10 steps

if answer = 2 then move 20 steps

if answer = 3 then move 30 steps

if touching the wall then turn around (change direction)

Code that models a push. A bigger push number makes the car move more steps.

Worked example

Let's debug the code

Kai tested his push game. A push of 3 made the car move only 10 steps, the same as a push of 1. Here are his lines:

if answer = 1 then move 10 steps
if answer = 2 then move 20 steps
if answer = 3 then move 10 steps

Test: he tried each push number and watched what happened. Push 1 → 10 steps. Push 2 → 20 steps. Push 3 → 10 steps. Something is wrong!
Find the bug: the line for answer = 3 says "move 10 steps". A bigger push should make a bigger change.
Fix: change it to "move 30 steps".
Test again: push 3 now moves 30 steps. The code works.

The bug was in the third line. It should say move 30 steps, so a bigger force makes a bigger change.

Just for fun

True fact: the hardest soccer kicks by pro players can send the ball faster than cars drive on a city street. Big force, big change!

Hands-on activity

Straw push test

You need: a drinking straw, a small crumpled paper ball (or a ping-pong ball), a ruler or tape measure, a flat table, and paper for a chart.

Put a piece of tape on the table as the start line. Put the ball on the line.
Predict: how far will a soft, a medium and a hard puff push the ball?
Blow one soft puff through the straw. Measure how far the ball rolled. Do it 3 times.
Do the same with medium and hard puffs.
Make a bar graph of your results (one bar for each puff size).
Change direction: set up a book standing up as a wall. Roll the ball toward it. What happens when it hits?
Change shape: squeeze a sponge, then let go. Squeeze play dough, then let go. Which springs back?

What to look for: harder puffs push the ball farther; the book wall pushes the ball back the other way; the sponge springs back but the dough stays squished.

Bonus coding: if you use Scratch or ScratchJr with a grown-up, build the push game and test it.

Safety: use your own straw and do not share it.

Your turn

Name the six changes a force can cause.

A goalie catches a puck. What change did the force cause?

What does "magnitude" mean?

Which will make a ball roll farther: a gentle tap or a hard kick?

A ball rolls toward a wall and bounces back. What exerted the force that changed its direction?

Two teams pull equally hard in tug-of-war. Does the rope move? What are the forces called?

You squeeze a sponge, then let go. Does it stay squished or spring back?

More practice: warm-up

A child kicks a ball that is not moving. What change happens?

You squeeze the brakes on your bike. What change happens?

You step on an empty juice box. What change happens?

Which is a bigger force: a gentle tap or a hard push?

In coding, what does "debug" mean?

More practice: on your own

In Noor's test, the medium puff went 30 cm and the soft puff went 12 cm. How much farther did the medium puff push the ball?

A hockey player hits a puck that is sliding toward him, and it zooms off toward the net. Name two changes the force caused.

Tug-of-war: Team A pulls hard and Team B pulls even harder. Which way does the rope move? Are the forces balanced or unbalanced?

Name one object that springs back after you squeeze it and one that stays changed.

In the push game code, what happens if you choose 2? What happens if the car then touches the wall?

Challenge

A sled is sitting still on flat snow. Two children push it from opposite sides, each just as hard. What happens? Then one child stops pushing. What happens now? Explain with balanced and unbalanced.

Kai wants his push game to also have a push of 4. Write the new line of code. How many steps should it be, if the pattern continues?

Plan a test to show that a bigger force changes an object's shape more. Use play dough and books. What will you change, keep the same and measure?

Check your understanding

Quiz

Which is not a change a force can cause?

A catcher's mitt stops a baseball. The force made the ball...

What does a bigger force usually do?

A ball bounces off a wall. What changed?

Both tug-of-war teams pull equally hard. The forces are...

Which object stays changed after you squeeze it?

Code says "if answer = 3 then move 10 steps", but it should move 30. What should you do?

For parents and teachers

Big idea: Ontario Grade 3 expectation C2.3 asks children to describe how different forces, including forces of different magnitude (size), can make an object start, stop, or change its direction, speed or shape. Bigger forces cause bigger changes. Balanced forces cause no change in motion; unbalanced forces do. The straw test is a fair test (A1.2) that uses measurement and bar graphs from math, and the push game is a simple coding model with testing and debugging (A2.1).

How to help: Play catch and talk about it: "Your throw started the ball, my glove stopped it." At the park, ask whether a gentle or strong push makes a swing go higher.

Common difficulties:

Thinking a force is needed to keep something moving at a steady speed. At this level, focus on forces causing changes; friction is usually what slows things down.
Missing that stopping needs a force. Something has to push against a moving object to stop it.
Confusing balanced (no change) with no forces at all. In tug-of-war, both teams are pulling hard; the forces just cancel out.

Coding note: you don't need a computer. Your child can "run" the code by moving a toy car along a ruler: 10 cm for push 1, 20 cm for push 2, and so on.