On Earth, gravity increases the speed of a falling object by about 9.8 m/s
every second that it falls.
In 0.4 second, its speed increases by (0.4 x 9.8) = 3.92 m/s.
If it just rolled off of the shelf at the beginning of that 0.4 second, then it
started from zero speed, and its speed after 0.4 second is 3.92 m/s.
Its velocity is 3.92 m/s downward.
