Science · Year 3 · Pushing and Pulling · Term 4

Unbalanced Forces: Changing Motion

Students will explore how unbalanced forces cause objects to start moving, stop, speed up, slow down, or change direction.

• ACARA Content Descriptions
• AC9S4U03

About This Topic

Unbalanced forces cause changes in an object's motion, such as starting to move, stopping, speeding up, slowing down, or changing direction. In Year 3, students explore this through pushes and pulls, directly addressing AC9S4U03. They explain why a rolling ball stops due to friction acting against its motion, compare effects of small versus large forces on speed, and design experiments to show direction changes, like deflecting a toy car.

This topic builds foundational physical science skills, linking everyday actions like kicking a ball or braking a bike to scientific principles. Students develop abilities to predict outcomes, control variables in tests, and communicate findings with evidence. It connects to the Pushing and Pulling unit by emphasizing that balanced forces maintain motion while unbalanced ones alter it, fostering precise observation and reasoning.

Active learning shines here because students experience forces firsthand through manipulatives. When they push objects, measure distances, or adjust ramps, abstract ideas become concrete. Collaborative experiments encourage discussion of results, helping students refine ideas and retain concepts longer than passive explanations.

Key Questions

1. Explain what causes a rolling ball to eventually stop.
2. Compare the effect of a small unbalanced force versus a large unbalanced force on an object.
3. Design an experiment to demonstrate how unbalanced forces change an object's direction.

Learning Objectives

• Explain why a rolling ball stops, identifying the force that opposes its motion.
• Compare the effect of a small unbalanced force versus a large unbalanced force on an object's speed.
• Design an experiment to demonstrate how an unbalanced force can change an object's direction.
• Classify pushes and pulls as examples of unbalanced forces that change motion.
• Predict the change in motion of an object when an unbalanced force is applied.

Before You Start

Identifying Pushes and Pulls

Why: Students need to be able to identify basic actions as pushes or pulls before understanding how these actions relate to forces.

Describing Motion (Start, Stop, Speed Up, Slow Down)

Why: Students must be able to describe changes in motion to understand how forces cause these changes.

Key Vocabulary

Unbalanced Force	A force that causes an object to change its motion, meaning it will start moving, stop moving, speed up, slow down, or change direction.
Friction	A force that opposes motion when two surfaces rub against each other, often causing objects to slow down and stop.
Motion	The process of moving or being moved; a change in position or place.
Direction	The path along which someone or something moves or develops; the way something is facing or pointing.

Watch Out for These Misconceptions

Common Misconception

A rolling ball stops because it runs out of energy or gets tired.

What to Teach Instead

Friction provides an unbalanced force opposing motion, gradually slowing the ball. Hands-on ramp tests with varied surfaces let students measure distances and see patterns, correcting this through evidence while peer talks refine explanations.

Common Misconception

Objects need a constant push to keep moving.

What to Teach Instead

Once moving, balanced forces allow steady motion, but unbalanced ones are needed to start or stop. Push-and-release car races reveal this, as students time glides and discuss friction's role, building accurate models collaboratively.

Common Misconception

Forces only come from hands or visible pushes.

What to Teach Instead

Invisible forces like friction and gravity act constantly. Balloon rocket pulls or ramp drops demonstrate these, with groups predicting and observing effects to expand force recognition beyond direct contact.

Active Learning Ideas

Ramp Races: Friction Forces

Provide ramps and balls or marbles. Students predict and test how different surfaces (carpet, smooth board, sandpaper) affect rolling distance, measuring with rulers. Groups discuss which unbalanced force causes slowing and record data on charts.

35 min·Small Groups

Push Comparisons: Force Strength

Use toy cars on flat surfaces. Students apply small, medium, and strong pushes, timing speed over a set distance with stopwatches. They graph results to compare how unbalanced force size changes motion.

30 min·Pairs

Direction Challenges: Obstacle Paths

Students build simple tracks with blocks and ramps, adding barriers to change a rolling object's direction. They test predictions, adjust barriers, and explain unbalanced forces involved in turns.

40 min·Small Groups

Collision Corners: Speed and Stop

Set up stations with balls of different sizes rolling into targets. Students observe speed changes and stops from collisions, noting unbalanced forces, then swap roles to collect class data.

45 min·Whole Class

Real-World Connections

• A hockey player uses their stick to apply an unbalanced force to a puck, changing its speed and direction to score a goal.
• Engineers design brakes for cars and bicycles. These brakes apply friction, an unbalanced force, to slow down or stop the vehicle safely.
• In a game of bowling, the bowler applies an unbalanced force to the ball, sending it down the lane to change the pins' motion.

Assessment Ideas

Quick Check

Ask students to hold a pencil in one hand and a book in the other. Instruct them to push the pencil with their finger, then push the book with the same finger but with more force. Ask: 'What did you observe about the pencil's motion? What did you observe about the book's motion? What caused the difference?'

Exit Ticket

Provide students with a scenario: 'A toy car is rolling across the floor. Suddenly, it hits a wall and stops.' Ask them to write two sentences explaining what happened to the car's motion and identify the force that caused it to stop.

Discussion Prompt

Present students with a toy car and a ramp. Ask: 'How could you use an unbalanced force to make the car change direction when it reaches the bottom of the ramp? What object or action would you use to create that force?' Facilitate a brief class discussion on their ideas.

Frequently Asked Questions

How do unbalanced forces change motion in Year 3 Australian Curriculum?

Unbalanced forces cause objects to start, stop, speed up, slow down, or change direction, per AC9S4U03. Students investigate through examples like friction stopping a ball or a push accelerating a car. Fair tests comparing force sizes help them predict and explain effects using evidence from measurements.

What experiments show unbalanced forces changing direction?

Design tracks with ramps and barriers for toy cars or balls. Students predict paths, test changes from angled pushes or obstacles, and measure deflections. This reveals how perpendicular unbalanced forces alter trajectories, with data logs supporting conclusions.

Common student misconceptions about unbalanced forces?

Many think objects stop from 'tiredness' or need constant pushes. Others overlook friction or gravity. Address with ramp races and collision activities where students observe, measure, and discuss data to replace ideas with evidence-based understanding.

Why use active learning for unbalanced forces in Year 3?

Active learning makes forces tangible as students push, roll, and collide objects, directly feeling effects. Group experiments like ramp tests build prediction skills and reveal patterns through shared data. Discussions during activities correct misconceptions on the spot, deepening retention over lectures alone.