Newton's FIRST lAW OF MOTION
In his Prncipia Newton formulated three laws that describe and predict the behavior of bodies in motion. Both Galileo and Cescartes recognized that rest and uniform motion were natural states of motion. Newton's first law, often referred to as the principle of inertia, is about uniform motion:
NEWTON'S FIRST LAW: A body remains at rest or moves along a straight line with constant velocity as long as no external force acts upon it.
If a body is in motion in a straight line at constant velocity (that is, its state of motion is a uniform state of motion), it continues to move along that line without changing its speed or direction as long as no force acts on the body. Another way of saying this is that its quantity of motion or its momentum remains the same. Thus the natural state of affairs in motion is for the body to continue whatever motion it has until an external force changes its motion.
