Posted by Anonymous on Sunday, August 28, 2011 at 2:55am.
What's the acceleration you're told to use in your class?
9.8m/s^2? 10m/s^2? 9.81m/s^2?
I'll go with 10 because I'm lazy, just substitute to whatever you guys are using.
The first ball (dropped from rest) will take 2 seconds to reach the ground :
20 = 5 x t^2 and you t=2
As such, the second ball will reach the ground in 1 second flat :
20 = Vinitial x t + 5 x t^2 where t=1
20 = Vinitial + 5
Answer : Vinitial = 15
Really, you just needed to use the equation you're given : x = x0 + v0*t + 1/2a*t^2
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