# Physics

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A ball is dropped from rest from a height of 20.0 m. One second later a second ball is thrown vertically downwards. If the two balls arrive on the ground at the same time, what must have been the initial velocity of the second ball?

• Physics -

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

Really, you just needed to use the equation you're given : x = x0 + v0*t + 1/2a*t^2

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