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Why do objects in motion continue moving in a straight line when no net force acts on them?

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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.

Objects in motion continue moving in a straight line at a constant speed because of the law of inertia, which is the first of Isaac Newton’s three laws of motion.

It is a common misconception that objects require a continuous force to remain in motion. In reality, a body maintains its state of motion—whether at rest or moving at a constant velocity—unless it is acted upon by an unbalanced force, such as friction or air resistance.12

Inertia is the inherent property of a body that resists changes to its motion, but it does not actively "push" an object forward. A moving body continues its path simply because there is an absence of an external force to slow it down, speed it up, or change its direction. In classical mechanics, rest and uniform motion in a straight line are considered the same state, as they differ only by the observer's frame of reference.3412