A dynamite blast at a quarry launches a rock straight upward, and 2.0 s later it is rising at a rate of 15 m/s. Assuming air resistance has no effect on the rock, calculate its speed (a) at launch and (b)4.2 s after launch

Answer 1

v = Vi - gt
when t = 2, v = 15
15 = Vi - 9.8*2
solve for Vi

then using that Vi solve for v at t = 4.2

Answer 2

a. is 34.6
so would b) be v=34.6-(9.8)(4.2)