Newton’s first law of motion states that:
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
This law is really only a definition of force. It says that when a body is acted upon by an external resultant force it will accelerate. An object resting will remain at rest, permanently, so long as nothing pushes or even pulls onto it. An object within motion will remain in motion, driving a straight line, eternally, till something forces or even pulls onto it.