Floating and Sinking Forces

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WALT (We Are Learning To)

• Demonstrate that any object lowered into water loses weight due to buoyancy.
• Represent the opposing forces of weight and buoyancy on floating objects using appropriate force arrows.

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Curriculum Alignment

Australian Curriculum:
Stage 4 – Year 7 Science, NSW Syllabus
Strand: Physical World
NSW Syllabus Code: SC4-10PW
Outcome:
The student describes the action of unbalanced forces on the motion of objects.

Content Focus:

• Investigate the effects of forces (including gravitational and buoyant) on objects.
• Use force diagrams to represent the direction and size of forces acting.

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Success Criteria

By the end of the lesson, students will be able to:

✅ Understand that objects lose apparent weight when submerged in water.
✅ Accurately draw and label force arrows to show opposing forces.
✅ Explain how buoyancy and weight act on objects submerged in water.

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Duration

60 minutes
Class size: 25 students
Grouping: Pairs with class discussion (whole group)

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Required Materials

• Spring scales
• Measuring cylinders or transparent containers
• Small objects of varying materials (plastic, wood, metal)
• Water trays
• Force arrow templates or whiteboard markers
• Force diagram worksheets (printed in dyslexia-friendly font: Lexend Deca or OpenDyslexic)
• iPads or laptops (for selected extension tasks)
• Laminated definitions of key vocabulary (buoyancy, weight, force, gravity)
• Force arrows (laminated physical arrows for kinesthetic activity)

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Lesson Breakdown

0:00–0:10 | Engage – Provocative Demonstration

Activity:
Teacher lowers a rubber duck into water while it is suspended from a spring scale.
Students observe the needle shift as the object seems to weigh less.

Teacher Talk:
"Why would this object appear to weigh less just because it’s in water? If the same duck weighed more in the air, what’s going on inside the water?"

Think-pair-share: Students predict and discuss in pairs.

Formative check:
Elicit a few responses highlighting ideas of ‘support from water’ or ‘water pushing up.’

0:10–0:25 | Explore – Small Group Experiment

Task: “What’s the Weight Underwater?”

• Students work in pairs.
• Each group selects 2-3 objects.
• Measure the object’s weight in air using spring scale.
• Lower it into water — measure the new reading.
• Record data in a table (worksheet provided).

Worksheet columns:
📌 Object | Weight in Air (N) | Weight in Water (N) | Difference (N)

Observe:
Students plot basic 'before and after' values.

🧠 Teacher circulates, supporting measurement accuracy and prompting key vocabulary use.

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0:25–0:35 | Explain – Force Diagram Construction

Mini Lesson:
Teacher walks through a visual explanation of two forces acting on a submerged or floating object:

• Downward force: weight
• Upward force: buoyant force

Anchor Model:
Simple diagram showing a floating object with arrows:

• Weight (↓) – labelled with Newtons
• Buoyancy (↑) – always opposes gravity up

Students use whiteboards or templates to draw force diagrams for:

1. A wooden block floating
2. A small stone sinking

✅ Encourage use of ruler and consistent arrow sizes
✅ Emphasise that balanced forces = floating
✅ Unbalanced forces = sinking

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0:35–0:50 | Elaborate – Kinesthetic Stations

Station Rotation:
(5 minutes per station in small groups)

🧪 Station 1: Buoyancy Body Challenge
Use physical arrow cards and attach to a human volunteer acting as the object. Whole group choreographs the direction/size of forces.

🎨 Station 2: Magnetic Force Match
Match correct force arrows to images of submerged objects.

📊 Station 3: Data Detectives
Reanalyse experimental data: Identify which objects would float/sink based on weight difference.

💬 Station 4: Roleplay Explanation
Pairs explain what’s happening using toy figures — student A = water, student B = object.

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0:50–0:58 | Evaluate – Think Alouds

Whole Class Discussion:
Use sentence starters to review learning:

• “When I put the object in water it…”
• “The force pushing up is called…”
• “If the object floats, the forces are…”

Exit Ticket Question:
“Draw a diagram showing the forces on a floating apple in water.”

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Differentiation Strategies

For Diverse Learners:

• Visual learners: Anchor charts, force diagram cards, clear labelling of arrows
• EAL/D students: Bilingual glossaries, picture cards
• Students with dyslexia: Lexend Deca font worksheets, minimal text per line, dyslexia-friendly colour coding (blue background)
• Hands-on/kinesthetic learners: Use of physical reinforcement activities, real-life experiments

For Advanced Learners (Extension):

• Investigate Archimedes’ Principle and real-world examples (e.g. boats, submarines).
• Independently calculate buoyant force using displacement volume.
• Use digital tools (e.g. PhET force simulation – optional pre-approved extension).

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Assessment Opportunities

• Formative: Observation during paired work, accuracy of diagrams, quality of discussion
• Summative: Exit ticket diagram and force explanation
• Differentiated Check-ins: Targeted questioning for Tier 2 & Tier 3 learners

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Vocabulary Focus

Word	Dyslexia-friendly flashcard definition
Buoyancy	The force that pushes things up in water
Weight	How heavy something is from gravity
Force	A push or a pull
Gravity	The force pulling things down

These should stay visible throughout the session on the classroom wall or screen.

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Teacher Reflection Prompts (Post-Lesson)

• Did most students grasp the weight change in water?
• How confidently could they draw and describe force diagrams?
• Was the kinesthetic activity engaging for reluctant learners or those with additional needs?
• How well did the extension learners investigate beyond the basics?

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This lesson is built to wow students with practical experience, cross-modal learning, and real-world relevance — encouraging scientific thinking, questioning and communication using Year-7-appropriate concepts and vocabulary.