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✦ Class 9 · Science
Gravitation and Floatation
Have you ever wondered why the Moon doesn't fall to Earth, but an apple always does?
What this lesson covers
Step by step, in this order.
- 1Ask about the Moon and an apple to set up the 'mystery'. What force makes things fall? What keeps the Moon up?
- 2Explain the Universal Law of Gravitation: every object attracts every other object. Introduce the formula F = G m1 m2 / r², the constant G (6.67 x 10⁻¹¹ N m² kg⁻²), and its units. Work through calculating F for two 100 kg bodies 1 m apart on the board.
- 3Explain that the force acts along the line joining the objects and follows the inverse square law: double the distance, and the force becomes a quarter.
- 4Introduce Earth's gravity: it's the Earth's pull on an object. Explain acceleration due to gravity 'g' = 9.8 m/s² near the surface, and that it does not depend on the mass of the falling object. Tell the story of Galileo's experiment.
- 5TRY IT Activity: Ask the student to drop a coin and a flat sheet of paper at the same time. Then, crumple the paper into a ball and drop it with the coin again.
- 6Define Mass as the amount of matter in a body (in kg), which is the same everywhere. Define Weight as the force of gravity on it (W = mg, in Newtons), which changes with place. Work through calculating the weight of a 50 kg student on Earth.
- 7Introduce Thrust as the force acting perpendicular to a surface. Define Pressure = Thrust / Area, with its SI unit Pascal (Pa) = 1 N/m². Work through calculating the pressure of a 60 kg person standing on one foot (area 0.02 m²).
- 8Explain that pressure in a fluid acts in all directions and increases with depth.
- 9TRY IT Activity: Ask the student to push an empty plastic bottle down into a bucket of water.
- 10Explain Archimedes' principle: the buoyant force equals the weight of the fluid displaced. Work through calculating the buoyant force on a body that displaces 500 g of water.
How it looks
A real board, not a video.
This board is from a different Science lesson — yours will look like this. She writes and draws while she explains, and stops to check you followed.
Doubts this lesson clears up
- Why don't we feel the Earth's gravitational pull on us all the time?
- If every object attracts every other object, why don't I get pulled towards my friend?
- Why does the acceleration due to gravity ('g') change with altitude?
- Why do some very heavy things float, while some light things sink?
- How do submarines manage to float and sink in water?
- Does pressure in a fluid only act downwards, or in other directions too?
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