# Universal Law of Gravitation

**Episode #2 of the course “Scientific laws and theories everyone should know”**

Newton’s equation, which first appeared in 1687, explains why everything stays firmly on the ground and how the Earth remains in orbit around the sun. It even explains how the sun was used to help send astronauts to the moon. The law simply states that all of the universe’s particles are attracted by gravitational force and explains the strength of such attraction.

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Essentially, force (F) between objects is relative to the amount of their masses (m1 and m2), divided by the square of the space between. The other part of the equation, G, represents the gravitational constant. In 2007, US scientists measured G at 6.693 × 10^(−11) cubic meters per kilogram second squared.

Newton developed the formula from observing the measurements given by prior astronomers. Stargazers took a long time to catalog the locations of planets and stars in the sky. By the 17th century, mathematician and astronomer Johannes Kepler discovered the geometry of these locations. By observing how Mars moved, Kepler thought that planets kept an elliptical orbit with the sun. Consequently, his three theories of planet motion helped astronomers calculate the positions for all future planets based on past data.

Kepler explained how planets moved, but Newton explained why by assuming an existing force was acting on the planets that allowed them to move around by themselves. Newton’s law signals that the pull of gravitational force for two objects lowers quickly, just as a light gets dimmer the further you are from it. This is known as an inverse square law.

Newton’s law of gravitation is plain, simple, and effective. If you plug in the numbers, you will be able to tell the positions of all of the planets, moons, and comets that you could ever see in your lifetime, even if they are on the other side of the universe.

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