A thrown object keeps moving
because air that is forced to the
side moves around behind to
push the object forward.
The medium provides force to
keep the object moving, and
resistance to slow it down. Force
can only be applied by a living
agent.
Speed is proportional to the
propulsive force and inversely
proportional to size [mass] and
resistance of the medium.
Objects are subject to only one
simple motion at a time.