Science · Year 3 · Pushing and Pulling · Term 4

Contact Forces in Action

Students will investigate forces that require direct physical contact, such as pushes, pulls, and friction.

• ACARA Content Descriptions
• AC9S4U03

About This Topic

Friction is the force that resists motion when two surfaces slide against each other. In this topic, students investigate how the texture of a surface, rough, smooth, wet, or dry, affects how much friction is produced. This aligns with ACARA's focus on how forces can be exerted by one object on another. Friction is a 'hidden' force that students use every time they walk, write, or ride a bike.

By testing different surfaces, students learn that friction can be both a help (stopping a car) and a hindrance (making it hard to slide a heavy box). In the Australian context, we can discuss the importance of friction in road safety, especially during 'wet weather' driving, or how First Nations people used friction to start fires. This topic is highly experimental and benefits from students making direct measurements. This topic comes alive when students can physically model the patterns of movement across various textures.

Key Questions

1. Explain how a door opens when you push it.
2. Compare the force needed to pull a toy car versus pushing it.
3. Design an experiment to measure the force required to move different objects.

Learning Objectives

• Compare the amount of force needed to push and pull identical objects across different surfaces.
• Explain how friction affects the movement of objects on surfaces with varying textures.
• Design an experiment to test how surface texture influences the force required to move an object.
• Identify examples of contact forces in everyday activities and explain their function.

Before You Start

Properties of Objects

Why: Students need to be familiar with different materials and their textures to investigate how surfaces affect forces.

Introduction to Forces

Why: Students should have a basic understanding that forces cause objects to move or change shape before investigating specific contact forces.

Key Vocabulary

Contact Force	A force that acts between objects only when they are touching each other.
Push	A force applied to an object that moves it away from the source of the force.
Pull	A force applied to an object that moves it towards the source of the force.
Friction	A force that opposes motion when two surfaces rub against each other.

Watch Out for These Misconceptions

Common Misconception

Heavy objects always have more friction.

What to Teach Instead

While weight does increase friction, the surface material is the primary factor. A heavy block on ice might slide easier than a light block on sandpaper. Testing this with different weights on the same surface helps clarify the relationship.

Common Misconception

Friction only produces heat.

What to Teach Instead

Heat is a byproduct of friction, but the main 'job' of friction is to resist motion. Using a 'slip and slide' analogy helps students see that the lack of friction is what causes sliding, not just a lack of heat.

Active Learning Ideas

Inquiry Circle: The Ramp Test

Groups set up a ramp and cover it with different materials (sandpaper, silk, bubble wrap, foil). They measure how far a toy car travels after leaving the ramp to determine which surface has the most friction.

45 min·Small Groups

Simulation Game: Shoe Designer

Students examine the soles of different shoes (sneakers, ballet flats, footy boots). They must 'pitch' a shoe design for a specific surface, like an icy path or a muddy field, explaining how the friction will help.

30 min·Small Groups

Think-Pair-Share: A World Without Friction

Students imagine a world where all friction suddenly disappeared. They think of one thing that would be impossible to do, share with a partner, and draw a 'frictionless' disaster scene.

20 min·Pairs

Real-World Connections

• Friction is essential for car tires to grip the road, preventing skids. Mechanics test tire tread depth to ensure sufficient friction, especially important in Australia's varied weather conditions.
• Firefighters use friction when sliding down poles to exit buildings quickly. The smooth pole allows them to slide, but friction between their clothes and the pole slows their descent safely.
• Australian Rules Football players rely on friction between their boots and the grass to make quick turns and stops on the oval. Different boot stud designs maximize grip on dry or wet turf.

Assessment Ideas

Quick Check

Provide students with a toy car and ramps covered in different materials (e.g., sandpaper, felt, smooth plastic). Ask them to push the car down each ramp and record how far it travels. Then, ask: 'Which surface created the most friction? How do you know?'

Discussion Prompt

Present students with images of everyday objects or activities (e.g., opening a door, a person walking, a book sliding on a table). Ask: 'What contact forces are being used in this picture? Is friction helping or hindering the action? Explain your reasoning.'

Exit Ticket

Students draw a simple diagram showing a push or a pull. They label the force and write one sentence describing how friction might affect the object's movement in their drawing.

Frequently Asked Questions

Why do we put oil in car engines?

Oil is a lubricant. It creates a thin layer between metal parts so they don't rub directly against each other. This reduces friction, which prevents the engine from getting too hot and wearing out.

Does air have friction?

Yes! It's called air resistance or 'drag'. When you move through the air, you are bumping into air molecules. This is why cyclists wear smooth helmets and crouch down to go faster.

What are the best hands-on strategies for teaching friction?

Using spring scales (force meters) to pull objects across different surfaces is excellent. It gives students a numerical value for the force, turning an abstract 'feeling' into hard data they can graph and compare.

How do First Nations Australians use friction?

Beyond fire-making, friction is used in tool-making. Rubbing a stone against a harder rock (grinding) uses friction to shape and sharpen axe heads or to grind seeds into flour on a coolamon.