Newton's first law of motion
law of inertia
definition
An object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.
example #1
the punching bag is at rest until a force from ones fist to put it in motion.
example #2
in this picture the football is at rest. the football will not move unless an outside fore changes that an applies a force.
example #3
the running back in the white jersey is moving in a straight line at a constant speed until the person in the red tackling him changes that, acting as the outside force.
Newton's First Law of Motion - Science of NFL Football