What is Newton's first law?
Newton's first law of motion, also known as the law of inertia, describes how an object or a body behaves while at rest or in linear motion. The law states that the body at rest or in motion will remain at rest or in motion at constant velocity, or speed in a straight line, unless an external force acts on it. For example, a ball simply placed on the ground will remain at rest until someone decides to kick it, which is when it will begin to move.
Larger and heavier objects, or rather objects that have more mass, have more inertia than objects with less mass, meaning more force is necessary to move them. Fast-moving objects also have more inertia that slow-moving objects. For example, a rock won't be moved by the wind on a windy day, but a piece of paper will easily be blown away; the rock has more mass than the piece of paper and therefore has more inertia.
Despite being called Newton's first law, in honor of the great scientist Sir Isaac Newton, the law of inertia was actually first deduced by Galileo Galilei, who, through his experiments with balls, concluded that a body in motion will continue moving unless a force stops it and causes it to come to rest. Newton simply refined Galileo's deduction and named it his first law of motion.