A projectile is launched from ground level with an initial velocity of V, feet per second. Neglecting air resistance, its height in feet t seconds after launch is given by s= - 16t2+ vot. Find the time(s) that the projectile will (a) reach a height of 400 ft and (b) return to the ground when vo is 128 feet per second.
(a) Find the time(s) that the projectile will reach a height of 400 ft when Vo = 128 feet per second. Select the correct choice below and, if necessary, fill in the answer box to complete your choice.