Here is a problem to help them memorize the situation.
A rifle is fired vertically down from a tall building. The
bullet, initially at rest, leaves the barrel with the velocity
of 300 m/s. The distance traveled inside the barrel is one
meter.
a) How large is the average acceleration inside the barrel?
b) How large is the acceleration outside the barrel (ignore
air resistance).
They must know that "free fall" is a situation in which
weight is the only acting force.
Later (when Fnet=m*a is known) you can add the third
question. Giving the bullet's mass (say 10 grams) ask:
c) How large would the acceleration be in water if the
water resistance force were 100 N?