Experience suggests that an object at rest remains at rest if left alone, and that an object in motion tends to slow down and stop unless some effort is made to keep it moving. However, Newton's first law gives a deeper explanation of this observation. The study of Newton's laws is like recognizing patterns in nature from which further patterns can be discovered. The genius of Galileo, who first developed the idea for the first law of motion, and Newton, who clarified it, was to ask the fundamental question: "What is the cause?"Thinking in terms of cause and effect is fundamentally different from the typical ancient Greek approach, when questions such as "Why does a tiger have stripes?"would have been answered in Aristotelian fashion, such as "That is the nature of the beast."The ability to think in terms of cause and effect is the ability to connect an observed behavior and the surrounding world.

Rather than contradicting our experience, Newton's first law says that there must be a cause for any change in velocity (a change in either magnitude or direction) to occur. This cause is a net external force. An object sliding across a table or floor slows down due to the net force of friction acting on the object. If friction disappears, will the object still slow down? The idea of cause and effect is crucial in accurately describing what happens in various situations. For example, consider what happens to an object sliding along a rough horizontal surface. The object quickly grinds to a halt. If we make the surface smoother by spraying it with talcum powder, the object slides farther. If we make the surface even smoother by applying oil on it, the object slides even farther. Extrapolating to a frictionless surface and ignoring air resistance, we can imagine the object sliding in a straight line indefinitely. Friction is thus the cause of slowing (consistent with Newton's first law). The object would not slow down if friction was eliminated.

Regardless of the scale of an object, whether a molecule or a subatomic particle, two properties remain valid and of interest in physics: gravitation and inertia. Gravitation is the attraction of one mass to another, while inertia is the ability of an object to resist changes in its motion—in other words, to resist acceleration. Newton’s first law is often called the law of inertia.

This text is adapted from Openstax, University Physics Volume 1, Section 5.2: Newton’s First Law.