Knowing an object’s initial position and velocity, and its subsequent acceleration, allows us to predict precisely its future position at any given time. This is the idea behind Newton’s clockwork universe theory. In general, predicting the future position of an object presents a mathematical challenge that can be solved with calculus. But in the special case where acceleration doesn’t change, a set of straightforward algebraic equations describes motion. Gravity near Earth’s surface provides an important case of motion with constant acceleration.