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Gravity and Orbits

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Lessons

1. Newton's Cannon: Falling Around the Earth (5 min): A ball fired sideways falls toward Earth, and the faster it goes, the farther it lands.
2. Moving in Circles: a = v²/r (7 min): Going round at a steady speed is still accelerating: the velocity keeps turning, and the change points at the centre.
3. Universal Gravitation: Inverse Square (7 min): Every two masses pull each other: F = G m1 m2 / r², with r measured centre to centre.
4. Orbit Speed and Period (7 min): In a circular orbit gravity is the centripetal force: GMm / r² = m v² / r gives v = √(GM / r).
5. Weightless Means Falling (7 min): At the space station gravity is still about 88% as strong as on the ground.
6. Ellipses and Kepler's Laws (7 min): Most orbits are ellipses with the Sun at one focus; a circle is the special case.
7. Escape Velocity (7 min): A planet sits in a gravity well: to leave for good you need enough energy to climb out of it.
8. Orbital Maneuvers: Slow Down to Catch Up (7 min): A forward (prograde) burn raises the far side of your orbit; a backward (retrograde) burn lowers it.
9. Tides: Gravity Stretches (7 min): The Moon pulls the near side of Earth more than the centre, and the centre more than the far side. The differences stretch Earth's ocean into two bulges.
10. Project: Mission to Mars (8 min): A Hohmann transfer to Mars is half an ellipse from Earth's orbit to Mars's orbit: about 8.5 months.

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