If you launch a bottle rocket into the sky with initial velocity 100m/s, how long will it take to come back to the launch height? ignore wind /air resistance

Explanation:

I assume the rocket is going straight up. Then we need to deal only with the vertical direction.

Start with the equation  

At t = 0, v₀ = 100 m/s.

At time t, at the height of the trajectory, v = 0.

a = g = -9.807 m·s⁻²

This is the time it takes to reach maximum height.

It will take the same time to reach the ground.

Thus,

Time of flight = 2t = 2 × 10.20 s = 20.4 s