15- If a projectile is fired vertically upward from a height of S feet above the ground with a velocity of y, fu/sec, prove that if air resistance is disregarded, its distance s(t) above the ground after f seconds is given by s(t) = - -gt + v.t + S,, where g is a gravitational (Hint: start with fit 2 constant. atz = d Ids dt ( 217 ) = -9