Newton's three laws of motion together form the foundation of classical mechanics, describing how and why objects move the way they do. The first law establishes that an object continues doing whatever it was already doing, staying still or moving at constant velocity, unless an outside force acts on it. The second law quantifies exactly how a given force actually changes an object's motion, depending directly on that object's mass. The third law adds that every force one object exerts on another is met by an equal, opposite force exerted back.
The first two laws connect force, mass and change in motion
Newton's first law, often called the law of inertia, states that an object at rest stays at rest, and an object in motion continues moving at a constant velocity in a straight line, unless some outside force acts to change that state. This directly overturned an older, more intuitive assumption that objects naturally tend to slow down and stop on their own; Newton's insight was that objects only slow down because of an active force, like friction, working against their motion, not because of some inherent tendency to stop. The second law then quantifies this relationship precisely: the force needed to change an object's motion by a given amount depends directly on that object's mass, meaning the same force produces a much smaller change in motion for a massive object than for a lighter one.
The third law means forces never come alone, always in matched pairs
Newton's third law states that whenever one object exerts a force on a second object, that second object simultaneously exerts an equal and opposite force back on the first. This isn't a delayed reaction or a separate causal event; the two forces occur simultaneously and are, in an important sense, two aspects of a single physical interaction between the two objects, not one force causing a separate second force afterward. This principle is exactly what explains phenomena like a rocket's propulsion, the rocket pushes exhaust gas one way, and that same interaction simultaneously pushes the rocket itself in the opposite direction, with no separate, additional force required to explain the rocket's own resulting motion.
What we're still unsure about
Newton's three laws, and their precise mathematical formulation, are extraordinarily well established classical physics, confirmed across centuries of consistent experimental and practical application at the scales of everyday objects. What's more a matter of well-understood limitation than genuine ongoing uncertainty is that Newton's laws, however reliable at ordinary speeds and scales, don't hold exactly at speeds approaching the speed of light or at the scale of individual subatomic particles, where relativity and quantum mechanics take over as the more accurate frameworks — this isn't an open question so much as a well-mapped boundary describing precisely where Newtonian mechanics remains an excellent, reliable approximation and where it stops being one.
This sits inside Newton's Laws of Motion, one of eight topics in Mechanics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.