Newton’s First Law of Motion is also known as the Law of Inertia. This law states that an object at rest will stay at rest until acted upon by an unbalanced force. This law also states that an object in motion will stay in motion until acted upon by an unbalanced force.
What does this mean?
Think of a pool table. The pool balls are slick. The balls remain at rest until acted upon by the cue stick or another ball – an unbalanced force. Once in motion, the pool balls remain in motion until they hit the bumper, drop into a pocket, or hit another ball – an unbalanced force. Hitting the other ball slows or stops the first ball, but causes the second ball to move in some way.
Think outer space. This is a place without friction – think of a pool table with the surface of an air hockey table. An object at rest in space will stay at rest. An object in motion in space will remain in motion forever.
Newton’s First Law is important because it is one of the basic assumptions of all physics.
More on this topic: Modeling Ships and Space Craft: The Science and Art of Mastering the Oceans and Sky by Gina Hagler — Part II – Evolution of Theory, Chapter Four – Hydrodynamic Theorists and Part III – Scale Model Testing Begins, Chapter Ten – It is Rocket Science