What happens when an object speeds up, slows down, or changes direction a) velocity b) time c) deceleration d) acceleration

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.

Variables,control,conclusion and finally scientific theory