Question

A stone is thrown with a velocity of 39.2 m/s making 30 degrees with the horizontal.the time after which it is at a height of 14.7m is

Answer 1

s = (v0sin30°)t - 1/2gt²
14.7 = (39.2 x 1/2) t - 1/2 x 9.8 x t²
14.7 = 19.6t - 4.9t²
4.9t² - 19.6t + 14.7 = 0
4.9t² - 14.7t - 4.9t + 14.7 = 0
4.9t (t - 3) - 4.9 (t - 3) = 0
(4.9t - 4.9)(t-3) = 0
so t can be 1 or 3sec