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Employed Johannes Kepler to work on the problem of the orbit of Mars. He observed the motions of the planets which lead to Kepler’s laws of planetary motion.
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Kepler’s laws of planetary motion:
1. Elliptical oribits around the Sun
2. The line joining the sun to a planet sweeps out equal areas in equal times
3. The square of the period T of the planet about the sun is proportional to the cube of the length of its semi-major axis r -
Visited Newton who asked about the trajectory of a body under the influence of inverse-square law forces directed towards a given centre. Helped Newton relate Kepler’s laws to centripetal forces.
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Law of universal gravitation (attraction is proportional to their masses)
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Created the torsion balance that is used to accurately measure the force of attraction between two masses (used by Cavendish)
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Calculated big G in order to determine Earth’s mass. He suspended dumbbells on a wire, with lead spheres at different distances nearby, then measured how much the dumbbells rotated in response to the attractive pull of gravity from the neighboring dumbbell.