Issac Newton was a brilliant scientist and mathematician. His greatest discovery was the law of gravity. He wanted to figure out what kept the moon traveling in its orbit around earth. He wondered if the moon was in motion, why didn't it fly off into space in a straight line?
Newton then saw an apple fall from a tree. He wondered if the same force that pulled the apple to the ground was tugging on the moon.
He realized that the force on the moon must be weaker, since it is so far away, he thought that the force would only be strong enough to bend the moon's motion into a circular path around the earth.
This was Newton's greatest insight. A single force that explained a falling apple and the orbiting moon around the Earth.
Newton stated the law of gravity simply. All physical objects had a force of attraction between them. The strength of the force varied on the masses and distance between each object. For example the moon and Earth tugged on each other, this happens because the moon revolves, or orbits, around Earth. In contrast, a falling apple would be dragged to the ground.
In 1687, Newton published a book that presented the law of gravity. His laws provided a physical explanation for what earlier scientists had discovered. Newton's laws changed people's picture of the universe. The same mathematical laws applied everywhere.
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