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How does Newton’s first law explain inertia?

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A diagram of a cylindrical vessel containing gas, with a movable piston held down by a weight. Countless small dots beneath the piston represent gas particles in rapid, random motion, illustrating Bernoulli's model of gas pressure. — physics As conceived by Daniel Bernoulli in *Hydrodynamica* (1738), gases consist of numerous particles in rapid random motion. He assumed that the pressure of a gas is produced by the direct impact of the particles on the walls of the container.

Newton’s first law, often called the law of inertia, states that a body will remain at rest or continue to move in a straight line at a constant speed unless acted upon by an external force.12

This law explains inertia by establishing it as the fundamental tendency of matter to resist changes to its state of motion. While common experience might suggest that objects naturally come to rest, the law clarifies that this observation is actually due to the presence of external, unbalanced forces—such as friction or air resistance—rather than an inherent desire for objects to stop moving.31

In this framework, inertia is a passive property; it does not actively cause motion but rather describes a body's resistance to any agency that attempts to change its velocity or direction. By defining this baseline behavior, the first law allows physicists to recognize when a force is acting on an object, serving as the essential starting point for classical mechanics.312