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How does Newton’s first law apply to an object moving at a constant speed?

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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.

Newton’s first law of motion, also known as the law of inertia, states that an object moving at a constant speed in a straight line will continue to do so unless it is acted upon by an unbalanced force.12

In classical Newtonian mechanics, this state of uniform motion is fundamentally linked to the state of rest; there is no significant distinction between the two, as they can be viewed as the same state of motion by different observers. If an object is observed to be moving at a constant speed in a straight line, the law implies that there is no net force acting upon it. Conversely, if an object that is not being pushed appears to come to rest, it is because external forces—such as friction or air resistance—are acting upon it to change its state of motion.321