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 ^_^

1 Answer
Mark C.
Oct 15, 2017

You only know initial velocity, u; acceleration, a; and displacement, s so you need to use #s = ut + 1/2a.t^2#

Explanation:

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

#s = 1/2a.t^2#

We need to rearrange for t

#t = sqrt((2s)/a)#

#t = sqrt(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.)

Related questions
Impact of this question
994 views around the world
You can reuse this answer
Creative Commons License