Why does the magnitude of acceleration remain constant?

Asked on Quora. My answer.

This happens when the net force is constant (read about Newton’s Second Law of Motion). A free falling object, that is one being impacted by the force of gravity only, is subject to a constant net force that is equal to the weight of the object. (mg). Since the gravity field (g) is roughly uniform close to the Earth’s surface this results in a constant net force and therefore a non-changing acceleration.

In uniform circular motion the magnitude of the acceleration also stays constant if the Centripetal force (another net force) has constant magnitude. However the direction of the acceleration will change though as the net force is perpendicular to the direction of motion of the object which forces a change in velocity but not speed.