Whenever the motion of an object changes . . . speeding up, or slowing down,
or changing direction . . . that change is called "acceleration". Acceleration is
produced by force on the object.
If there is no force on the object, then there is no acceleration. That means that
its motion doesn't change. The object remains in constant, uniform motion ...
moving with steady speed, in a straight line.
No force is necessary to keep an object moving, only to change its motion.
