I keep trying to get this right but I keep falling short. Here's the problem:(adsbygoogle = window.adsbygoogle || []).push({});

A hoodlum throws a stone vertically downward with an initial speed of 19.0 m/s from the roof of a building, 39.0 m above the ground. (a) How long does it take the stone to reach the ground? (b) What is the speed of the stone at impact?

My professor has given us a number of formulas to use for motion problems and they are as follows:

Vf = Final Velocity, Vi = Initial Velocity, a = acceleration, t = time, d = displacement

First Formula: Vf^2 = Vi^2 + 2ad

Second Formula: d = Vi(t) + (a/2)(t^2)

Third Formula: Vf = Vi + at

Which formula could I use to solve this problem? For part a based on what my textbook indicates it'd seem that the second formula will do it, but when I plug in the data that's given I end up with:

39m = 19t - 4.9t^2

But as you can see this doesn't give me a single variable. (Well it does, t, but I end up with a quadratic equation).

Do I just solve the quadratic or am I doing something wrong? And if I'm doing something wrong, am I using the right formula of the 3 or do I need to use a different one? Please help! Thanks!

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# Homework Help: Stuck on Free-Fall Acceleration problem

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