| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Physics |
| Lesson Topic: understand that, for a point outside a uniform sphere, the mass of the sphere may be considered to be a point mass at its centre |
Learning Objective/s:
- Describe the shell theorem and its implication for external points of a uniform sphere.
- Explain why a uniform sphere can be treated as a point mass at its centre for external gravitational calculations.
- Apply the point‑mass formula to calculate gravitational force and field for planets, stars or laboratory spheres.
- Solve problems involving gravitational force inside and outside a uniform sphere, recognising the linear variation inside.
- Identify and correct common misconceptions about mass distribution and gravitational force.
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Materials Needed:
- Projector or interactive whiteboard
- PowerPoint/slide deck with shell‑theorem diagrams
- Printed worksheet with practice questions
- Calculator or computer algebra system
- Small spherical objects (e.g., ball bearings) for demonstration
- Ruler or measuring tape for scaling diagrams
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Introduction:
Begin with a striking image of Earth’s gravity pulling an apple, asking students how we can calculate that force without knowing the Earth’s internal structure. Recall Newton’s law of universal gravitation for point masses and the success criteria: students will be able to treat any uniform sphere as a point mass when evaluating the field outside it. We will also link this idea to everyday examples such as satellite orbits and weight measurements.
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Lesson Structure:
- Do‑now (5’) – Quick mental‑recall quiz on F = G m₁ m₂ / r² and previous work with point masses. (Check understanding)
- Mini‑lecture (10’) – Introduce the shell theorem with diagram, explain cancellation of perpendicular components, and state the external‑field result. (Conceptual)
- Demonstration (7’) – Use two small spheres and a larger uniform sphere model to illustrate that the external field depends only on total mass; students predict the outcome. (Inquiry)
- Guided practice (15’) – Work through the Earth‑weight example and a sample worksheet problem; students calculate in pairs while the teacher circulates. (Application)
- Check for understanding (8’) – Exit‑ticket: write the formula for gravitational field outside a uniform sphere and one sentence why the interior distribution is irrelevant. (Formative)
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Conclusion:
Summarise that for any point outside a uniform sphere the entire mass can be considered concentrated at the centre, simplifying calculations. Remind students of the exit‑ticket answer and clarify any lingering misconceptions. Assign homework: complete the remaining worksheet problems and prepare a short explanation of how the shell theorem applies to planetary orbits.
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