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.)

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