# A ball is free falling from a 60 m tall building. What is the time it will take to hit the ground ?

## What should I use here ? Which rule ? With steps please, thank you ^_^

Oct 15, 2017

You only know initial velocity, u; acceleration, a; and displacement, s so you need to use $s = u t + \frac{1}{2} a . {t}^{2}$

#### Explanation:

Fortunately, u is zero so the equation is much easier to solve. It now reduces to:

$s = \frac{1}{2} a . {t}^{2}$

We need to rearrange for t

$t = \sqrt{\frac{2 s}{a}}$

$t = \sqrt{\frac{120}{9.81}}$

$t = 3.5 s$

Perhaps the lesson is really in the technique you use to start - write out a table with the factors s, u, v, a & t and fill in the data you know. One unknown (you are told to find) and one irrelevant factor will be missing, put a question mark for the unknown and a dash for the one you can ignore. Each SUVAT equation misses out one of the factors, so the equation you need becomes apparent (quite quickly with practice.)