A ball is thrown upward with velocity 30 ft/s on the roof of a 50 ft building - find the total time until the object meets the ground.(adsbygoogle = window.adsbygoogle || []).push({});

Heres my problem - I used the equation

x = (xsub0) + (vsub0)t + 1/2(a)t^2

I set xsub0 = 50, vsub0 = 30, and a = -9.8

I solved for t when x = 0, but the book gave a totally different answer.

Then I went back and used the equation given in the book:

[tex]\Delta[/tex]x = (vsubnot)[tex]\Delta[/tex]t + 1/2(a)([tex]\Delta[/tex]t)^2

I solved for [tex]\Delta[/tex]t when [tex]\Delta[/tex]x = 50, and my answer agreed

with the textbook solution...

My question is, why does the 2nd equation work in this situation, and how do I know when it is appropriate to use which?

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# Homework Help: Kinematics question - which equation to use?

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