Newton's First Law of Motion

Newton's first law of motion states: a body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by some external force acting on it. Put another way, in the complete absence of any net external force, a body at rest stays at rest forever, and a body already moving keeps moving at exactly the same speed, in exactly the same straight-line direction, forever -- it neither speeds up, nor slows down, nor curves away from its path on its own.

This law is often called the law of inertia, because it is really just the everyday idea of inertia (Section 4.1) written as a precise, testable statement. It runs directly against a very common everyday intuition -- that a continuously applied force is needed just to keep something moving at constant speed, since a rolling ball or a coasting cart does, after all, eventually slow down and stop. The resolution is that such everyday moving objects are never actually free of external force: friction with the ground and resistance from the air are both external forces constantly acting on them, opposing their motion and slowing them down. If those resistive forces could genuinely be reduced to zero -- as they very nearly are for, say, a puck sliding on a highly polished ice rink, or a spacecraft coasting far from any planet or star -- the object would continue moving in a straight line at constant speed for as long as no other force acted on it, exactly as the first law describes. …