For this problem, assume that the acceleration of gravity is -10 m/s², and that all friction effects can be neglected. A ball is thrown upward at a velocity of 20 m/s. How fast is it moving when it returns to the ground?
a. zero
b. 10 m/s
c. -10 m/s
d. -20 m/s
e. +20 m/s.
