determine when the projectile will first reach a height of 80 ft, and how many seconds later will it again be 80 feet?

Question

h(t)= -16t² + vt + h₀

 

Velocity: 235ft/s

Answer 1

We need one more piece of information. From what initial height was the projectile launched; i.e. what is h₀?  So far we have

h(t) = -16t² + 235t + h₀

Once you know h₀, set h(t) = 80 and solve the resulting quadratic equation for t. There should be 2 solutions. The lower t would be when it reaches 80 ft on the way up and the higher t will be when it reaches 80 ft on the way down. But you need to know h₀. The difference between the 2 times is the number of seconds in between the times it reaches 80 feet.