Newton's second law describes the motion of the center of mass of most objects that we encounter in everyday life, including the complex motion of a spoon when we eat, the acceleration of a car, and even the trajectories of planets and stars. Only if objects move extremely fast, do we need to replace Newton?s second law with Einstein?s theory of relativity, and only if objects are extremely small, do we need to use the laws of quantum mechanics. Newton?s second law states that the net force F on an object equals the product of its mass m and its acceleration a: F 5 m 3 a. Unfortunately, the acceleration of the center of mass of an object is hard to measure. It is the rate of change of the velocity, where the velocity of the center of mass is its change in position per unit of time. Most of the time, we cannot determine the acceleration or the velocity directly but measure the location of the surface of the object at fixed time intervals. We use this data to estimate the position of the center of mass and then compute its acceleration. This is a complicated procedure.