# Physics

A rock is thrown vertically up into the air. It reaches a maximum height 15m above point where it was released. What is its initial velocity?

a) 17 m/s
b) 26 m/s
c) 14 m/s
d) 20 m/s

I know the answer is a) 17 m/s and I got it wrong thinking it was d) 20 m/s but I can't seem to figure out how to find the answer. Am I using the correct formula? Vi = Vf - (a x t)?

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1. Ignoring air resistance, equate potential and kinetic energies:
mgh = (1/2)mv²
Cancel m,
v=√(2gh)
=√(2*9.8*15)
= 17.1 m/s

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posted by MathMate

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