Floating and Sinking

Curriculum Links

• Subject: Science
• Level: KS3 (Year 7)
• National Curriculum Area: Physics – Forces and Motion
• Specific Focus: Buoyancy, density, and upthrust in liquids

Lesson Objectives

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

1. Define and explain the concepts of floating, sinking, density, and upthrust.
2. Predict and test objects to determine why they float or sink.
3. Apply their understanding of buoyancy to design a ‘Mermaid’s Raft’ that can support weight.

---

Lesson Structure (75 Minutes)

1. Hook – The Mermaid’s Problem (10 mins)

• Storytelling: Introduce students to the imaginary scenario – A mermaid has lost an important object in the ocean and needs to build a safe floating platform to retrieve it. How can we help her?
• Question Prompt: Why does a mermaid float, while her treasure chest sinks? Quickly gather initial student thoughts on floating and sinking.
• Demonstration: Fill a large transparent container with water. Drop objects (e.g., a coin, a plastic bottle, and a small wooden block) and ask: Why does each one behave differently?

---

2. Explanation – The Science of Floating and Sinking (15 mins)

• Discuss density using the equation:
  • Density = Mass ÷ Volume
  • Compare the density of water vs. different materials.
• Explain upthrust (buoyant force):
  • Objects float if upthrust is greater than their weight.
  • Use the example of a boat vs. a stone.
• Link this to mermaids:
  • If mermaids were real, what properties of their bodies might allow them to float or dive?

---

3. Hands-On Experiment – Will It Float? (20 mins)

Students work in pairs with a tray of materials to predict and test buoyancy.

Materials (per group):

• Clay
• Aluminium foil
• Sponge
• Plastic bottle (empty and filled)
• Coins
• Wooden lolly sticks
• Small polystyrene ball
• Large water containers/buckets

Instructions:

1. Predict whether each object will float or sink.
2. Test each item in water and record observations.
3. Answer: What patterns do you notice? How does shape affect floating?

Discussion Prompt:

• Why does a heavy boat float but a small stone sink?
• Can we change an object's shape to make it float?

---

4. Engineering Challenge – Build a Mermaid’s Raft (25 mins)

Task:

• Groups design and build a floating ‘raft’ using materials provided.
• The raft must support weight (a small toy or coin stack).
• Groups test & refine their designs in water.

Materials (per group):

• Wooden lolly sticks
• Elastic bands
• Clay
• Aluminium foil
• Tape
• Small plastic lids

Success Criteria:

• The raft must float without tipping.
• It should support a small object for at least 10 seconds.

Debrief:

• Why did some rafts perform better?
• How do boat designers use these principles in real life?

---

5. Plenary – Exit Reflections (5 mins)

• Quick quiz:
  • What is density?
  • What is upthrust?
  • Name one object that floats and one that sinks.
• Final Thought: If you were designing a mermaid’s treasure chest, what material would you use to make sure it sinks but can be retrieved easily?

---

Assessment Opportunities

• Observation during the floating/sinking experiment
• Group discussions and reasoning
• Effectiveness of the raft in the engineering challenge
• Exit quiz responses

Differentiation

• Support: Provide sentence starters for discussions. Use real-life objects to illustrate concepts clearly.
• Challenge: Ask students to modify the density of an object (e.g., altering foil shapes, adding air pockets).

Resources Needed

• Large water containers/buckets
• Variety of materials for testing buoyancy
• Craft materials for raft-building

---

Teacher Reflection

• Did students grasp the connection between density and floating?
• Were they engaged in hands-on activities?
• How well did students apply their learning in the raft-building challenge?

A memorable and interactive lesson that brings science to life through a combination of storytelling, experiments, and engineering! 🌊 🧜‍♀️