having trouble setting this up and isolating the key information to solve this problem. here it is.(adsbygoogle = window.adsbygoogle || []).push({});

question: A ball is thrown straight up from the edge of the roof of a building that is 40.0 m high, and a second ball is dropped from the roof 2.00 s later. What is the initial velocity of the first ball if both balls hit the ground at the same time?

answer: 15.6 m/s

=======================

my effort: alright i started by trying to apply these formulas..

v0 = v + at

....

x = x0 + v0t + 1/2at^2 --> tried using this one.

....

v^2 = v0 + 2a(change x)

i think i'm not really using x in the equations so i'll change them to y because were doing verticle drops here.

...

also, i must be forgetting steps here.

i thought that the middle formula would work best, so i plugged in what i knew

and used gravity (9.80)

what we know:

x = 0 (because x final hits the ground)

x0 = 40.0m because thats where both balls are when there first released.

v0 = thats the variable were trying to find

t = 2seconds ? NOT SURE if we can use 2 seconds here.

a = i used gravity for this variable (9.80) i'm not sure why we use - or + though, maybe a quick explanation here please.

....

okay. so as i said, i used the 2nd equation: x = x0 + v0t + 1/2at^2

i plugged the answers in: and calculated: 10.2 m/s

i think i'm missing a big step here.

please help. thanks.

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# Homework Help: Ball, gravity, velocity problem

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