Description: Newton’s First Law states that every object continues in a state of rest unless acted upon by a force. An unmoving object and an object moving at uniform speed are really the same thing.
Key Terms: Rest, net force, speed, uniform speed, velocity

The tendency of things to resist changes in motion was what Galileo called inertia. Newton refined Galileo’s idea and made it his first law, appropriately called the law of inertia. From Newton’s Principia (translated from the original Latin):

Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force.

The key word in this law is continues: An object continues to do whatever it happens to be doing unless a force is exerted upon it. If it is at rest, it continues in a state of rest. This is nicely demonstrated when a tablecloth is skillfully whipped from under dishes on a tabletop, leaving the dishes in their initial state of rest. This property of objects to resist changes in motion is called inertia.

If an object is moving, it continues to move without turning or changing its speed. This is evident in space probes that continually move in outer space. Changes in motion must be imposed against the tendency of an object to retain its state of motion. In the absence of net forces, a moving object tends to move along a straight-line path indefinitely.