Newton applied his laws of motion to the problem of the motions of the planets. He assumed that it was gravity that held them in their orbits,

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Just need to know if my answer or right or wrong. Thanks! :)

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Your answer is correct. Newton indeed applied his laws of motion to explain the motions of the planets and assumed that gravity was the force that kept the planets in their orbits. Newton's laws of motion and his law of universal gravitation were revolutionary in understanding and predicting the behavior of celestial bodies.