Newton
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Kepler's laws explained how the planets orbit the sun. Isaac Newton asked why the planets moved in that way. He compared a planet to a rolling ball. He figured that because a planet does not follow a straight path, an outside force must cause it to curve. If there wasn't force acting on it, the Earth would continue in a straight path forever (Inertia). Newton identified this force as gravity, the attractive force that exists between all objects in the universe. The gravitational pull of the sun keeps planets in orbit around it.