Britannica Sources used in this answer

Why must a net force act on an object for its velocity to change?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Title page of Isaac Newton's Philosophiae Naturalis Principia Mathematica, 1687, featuring large Latin text and publication details in a bordered layout. — Newton’s laws of motion The title page of Isaac Newton's *Philosophiae Naturalis Principia Mathematica* (1687; *Mathematical Principles of Natural Philosophy*), the work in which the physicist introduced his three laws of motion.

According to Newton’s first law of motion, also known as the law of inertia, an object will remain at rest or continue to move at a constant speed in a straight line unless it is acted upon by a force.12

A change in velocity—which involves a change in speed, direction, or both—requires a net force because of the inherent property of inertia, which is the tendency of an object to maintain its current state of motion. In classical Newtonian mechanics, if no net force acts on a body, its momentum remains constant, meaning it cannot accelerate.1234

Newton’s second law quantifies this relationship, stating that the time rate of change of an object's momentum is equal to the net force applied to it. Therefore, if a body is observed to be accelerating (changing its velocity), it is a direct consequence of a net force acting upon it; conversely, if there is no net force, there is no change in velocity.2