Question

The height, h, in feet of a golf ball above the ground after being hit into the air is given by the equation, h = -16t^2 + 64t, where t is the number of seconds elapsed since the ball was hit. How many seconds does it take for the golf ball to hit the ground?

Answer 1

The ball is on the ground whenever the height is 0:


`-16t^2+64t=0\implies-16t(t-4)=0\implies t=0,t=4`

Obviously,
`t=0` is the time when the ball is thrown, so ignore that. It thus takes the ball
`t=4` seconds to hit the ground after it's been launched.