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# ball thrown up, another ball dropped, when do they meet ?

A ball is thrown upward from the ground with an initial speed of 25 m/s; at the same instant, a ball is dropped from rest from a building 15 m high.  After how long will the balls be at the same height ?

g =  - 9.81 m/s.s

This is high school straight line free fall motion question.

Attached is the equation I used to solve this problem.

I let the final positions of the two motions equal to each other.  That gave me time of 3/5 sec.

I made origin 15 m above ground.  So, ball was thrown from initial position of  -15m.  Ball was dropped from initial position of 0 m .

Is my answer of 3/5 sec. correct ?
motion-equation.png
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naseeam
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1 Solution

Commented:
yes
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Commented:
note that the answer does not depend on g
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Author Commented:
Thanks for mentioning this point.  I see the  g  cancels out.
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Commented:
You can also see from the situation that g has the same effect on both balls, so the distance between the balls would not be affected.
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Author Commented:
It's intuitive!

time to get to final position for any  g  would be same.
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