Science · Year 7 · Forces in Motion · Term 2

Introduction to Forces

Students will define force as a push or pull, identify different types of forces, and understand how forces cause changes in motion.

• ACARA Content Descriptions
• AC9S7U04

About This Topic

Gravity as a Force introduces students to one of the fundamental non-contact forces of the universe. In Year 7, the focus is on gravity as a pull that exists between any two objects with mass, though we only notice it when one of those objects is as large as a planet. Students learn to distinguish between mass (the amount of matter in an object) and weight (the force of gravity acting on that mass), a distinction that is crucial for further studies in physics.

This topic connects to Earth and Space science by explaining why planets orbit the sun and why we have tides. In the Australian Curriculum, students are encouraged to look at how gravity affects everything from the flight of a boomerang to the way water flows in our river systems. This topic comes alive when students can physically model the patterns of gravitational pull and use simulations to see how changing mass or distance affects the force.

Key Questions

1. Differentiate between contact and non-contact forces with examples.
2. Explain how forces can change an object's speed, direction, or shape.
3. Analyze the net force acting on an object at rest or in constant motion.

Learning Objectives

• Classify given forces as either contact or non-contact forces.
• Explain how a net force causes an object to accelerate, decelerate, or change direction.
• Analyze how applying a force can change an object's shape.
• Compare the effects of balanced and unbalanced forces on an object's motion.

Before You Start

Properties of Matter

Why: Students need a basic understanding of matter and its properties to comprehend how forces can affect an object's state or shape.

Introduction to Motion

Why: Understanding concepts like speed and direction is essential before exploring how forces cause changes in motion.

Key Vocabulary

Force	A push or a pull that can cause an object to change its motion, speed, direction, or shape.
Contact Force	A force that acts between objects that are physically touching each other, such as friction or a push.
Non-Contact Force	A force that acts on an object without physical contact, such as gravity or magnetism.
Net Force	The overall force acting on an object when all individual forces are combined. It determines the direction and magnitude of the object's acceleration.
Friction	A force that opposes motion between two surfaces that are in contact. It can slow down or stop moving objects.

Watch Out for These Misconceptions

Common Misconception

There is no gravity in space or on the Moon.

What to Teach Instead

Students often think astronauts float because there is 'no gravity'. Explain that gravity is everywhere; it's just weaker further from Earth. On the Moon, gravity is about 1/6th of Earth's. Using videos of the Apollo moonwalks helps students see that things still fall, just more slowly.

Common Misconception

Heavier objects fall faster than lighter ones.

What to Teach Instead

This is a classic Aristotelian error. In a vacuum, all objects fall at the same rate. On Earth, air resistance complicates things. A hands-on experiment dropping a crumpled piece of paper and a flat one helps students see that it's air, not gravity, that causes the difference.

Active Learning Ideas

Simulation Game: Gravity on Other Worlds

Using a digital simulator or a set of 'scaled' weights, students compare how high they could jump or how much a 1kg bag of flour would 'weigh' on the Moon, Mars, and Jupiter. They record their findings and discuss why the weight changes but the mass doesn't.

35 min·Small Groups

Think-Pair-Share: The Falling Objects Mystery

The teacher drops a heavy ball and a light ball (of similar size) simultaneously. Students predict which will hit first, observe the result, and then work in pairs to explain why gravity pulls on them equally despite their different masses.

20 min·Pairs

Inquiry Circle: Mapping the Pull

Groups use spring balances to measure the weight of various objects. They then use a 'gravity well' (a lycra sheet stretched over a hoop with a heavy ball in the center) to visualize how mass curves space and attracts smaller objects.

45 min·Small Groups

Real-World Connections

• Engineers design roller coasters by calculating the forces of gravity, friction, and air resistance to ensure a safe and thrilling ride for passengers.
• Sports scientists analyze the forces applied by athletes during activities like kicking a football or throwing a javelin to improve technique and performance.
• Mechanics use their understanding of forces, including friction and tension, to diagnose and repair issues with vehicle brakes and steering systems.

Assessment Ideas

Exit Ticket

Provide students with images of different scenarios (e.g., a book on a table, a magnet attracting paperclips, a car braking). Ask them to identify the main force(s) acting in each scenario and classify them as contact or non-contact.

Quick Check

Ask students to stand up and push against a wall. Then ask: 'Are you applying a force? Is the wall moving? What does this tell us about the forces acting on the wall?' Discuss the concept of balanced forces.

Discussion Prompt

Pose the question: 'Imagine you are pushing a box across the floor. What happens to the box if you push harder? What happens if friction suddenly increases? What if you push in a different direction?' Guide students to explain changes in motion due to varying forces.

Frequently Asked Questions

What is the difference between mass and weight?

Mass is the amount of 'stuff' or matter in an object, measured in kilograms (kg), and it stays the same everywhere. Weight is a force caused by gravity pulling on that mass, measured in Newtons (N). If you go to the Moon, your mass stays the same, but your weight decreases because the Moon's pull is weaker.

Why don't we feel the gravity of other people?

Gravity depends on mass. While every object with mass has a gravitational pull, the force is extremely weak unless the object is massive, like a planet. The pull between two people is so tiny that it's completely overwhelmed by the friction of your shoes on the floor and the Earth's massive pull.

How can active learning help students understand gravity?

Gravity is invisible and often counter-intuitive. Active learning, such as using spring balances or 'gravity well' models, allows students to feel and see the effects of the force. By predicting and then testing how objects fall, students confront their misconceptions directly, leading to a more permanent shift in their understanding.

Who 'discovered' gravity?

While Isaac Newton is famous for formulating the law of universal gravitation in the 17th century, humans have always understood its effects. First Nations Australians, for example, have used an understanding of gravity and aerodynamics for thousands of years to design tools like the returning boomerang and the woomera.