Transcription: Newton's first law of motion says that an object travels at a constant velocity unless acted upon by an outside force. But Newton's greatest discovery was that this law worked not only on objects on Earth, but on the planets above. Newton figured that if the moon was traveling in a circle around the Earth, there must be a force acting on it. Otherwise, by his first law, it would travel in a straight line. That force was gravity. Here's an interesting result. If you were standing on the moon where there's no air resistance, and threw a ball horizontally, no matter how hard you threw it, gravity ...
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