The idea that objects only change their velocity due to a force is encapsulated in Newton's first law.


Newton's first law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force.

Newton's second law of motion is F = ma, or force is equal to mass times acceleration. 

Newton's third law: If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A.