# Gravity

The moon is 240,000 miles from earth-center, or 60 times as far as we are from center

About 50 years before Newton formulated the law of gravity, Johanne Kepler (1571-1630) had found some rules that could be used to calculate the orbits of the planets

Kepler's third law states the relationship between a planet's average distance from the sun and the time required for the planet to complete one orbit

Newton argued that if a cannon could be placed on a hill high enough to be outside the earth's atmosphere, and if the cannon could be made powerful enough, a cannonball could be placed in orbit around the earth.

The law of gravity included a symbol for proportionality. In order to use this relationship for solving problems, we must remove the proportionality by placing a constant in the expression

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