Question

An object is thrown vertically from a height of `9 m` at `24 m/s`. How long will it take for the object to hit the ground?

Answer 1

I found: `5.2s`

Explanation:

I would suppose that the velocity is in the upwards direction because it is positive and so in:

`y_f-y_i=v_it+1/2at^2`
with:
`y_f=0` the ground
`a=g` downwards.

we have:

`0-9=24t-1/2*9.8t^2`
`4.9t^2-24t-9=0`

Using the Quadratic Formula:
`t_(1,2)=(24+-sqrt(576+176.4))/9.8=`
`t_(1,2)=(24+-27.4)/9.8=`
`t_1=5.2s`
`t_2=-0.3s`