He invented calculus (Leibnitz in Germany also invented it). Newton used calculus to show that the curved path of a planet's orbit is due to the inward acceleration of the planet towards the Sun, which is exactly balanced by the inward force of gravity at every instant. This enabled him to explain Kepler's law of planetary motion in terms of the new theory of particle dynamics.
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Isaac Newton and Gottfried Wilhelm Leibniz
No, Einstein did not invent calculus, Isaac Newton did. Liebniz did as well at around the same time and there was a major debate on who invented calculus first, but I think the more popular answer is Isaac Newton. Put Leibniz–Newton calculus controversy into wiki for an artcile about this debate.
Newton "invented" "applied mathematics" and calculus.
He did better than that.He invented it.
Calculus, both differential and integral.