A rock is thrown upward from the edge of a 40m high cliff with an initial speed of 15m s−1. Neglect air resistance and take g=9.8m s−2. Determine the speed with which the rock hits the ground at the foot of the cliff.
(a) 37.2m s−1
(b) 31.8m s−1
(c) 23.6m s−1
(d) 28.0m s−1