# A History of Gravity: Aristotle through Cavendish.

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A History of Gravity: Aristotle through Cavendish

Ancient and Pre-Modern Theories Aristotle’s theory of four elements – Earth moves to earth, – heavier objects fall faster

Galileo and the Scientific Revolution: 1564-1642 Ramp experiments: showed bodies of any mass have the same acceleration Found period of pendulum independent of mass Leaning Tower of Pisa Story is a Story...

Isaac Newton: 1643-1727 Puts it all together Worked from astronomers data (like who?) Asked what made moon orbit the earth Apple on head story: The same force acts on objects and planets! F g = (Gm 1 m 2 )/R 2 Big idea, though he remained religious

Weighing the Earth: Henry Cavendish (1731-1810) Set up sensitive apparatus designed by a friend Wanted to “weigh the earth” Measured Mass, Force, distance Could figure out G Got within 1% of modern accepted value Force was 1x10 -7 N

A modern day version of the apparatus:

Gravity Three variables affect the force of gravity – Mass of each object – Distance between objects How do these affect the force of gravity? – As the masses of the objects increase… – As the distance between them increases…

Force of gravity between two objects of masses m 1 and m 2 separated by a distance r. universal gravitational constant

Force of gravity between two objects of masses m 1 and m 2 separated by a distance r. Universal gravitational constant Gravitational field strength due to some mass m. (Usually something big, like a planet.) The r is the distance to the center of the planet if you’re on it.

Image References Galileo portrait by Giusto Sustermans Newton portrait by Godfrey Kneller En.wikipedia.org

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