A ball is thrown vertically upwards with a velocity 'u'. The velocity with which it falls to the earth again is:

For vertically upward motion: g is negative as it opposes the motion.

v = u – gt

0 = u – gt

t = u/g                 …..(1)

For vertically downward motion: g is positive as it is in the direction of motion.

v = u + gt

v = 0 + gt

t = v/g                 …..(2)

From equation 1 and 2

v = u