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## Homework Statement

a man throws a stone vertically upward with initial speed of 20 m/s from a building with height 54 m a)how long does the stone take to reach 40 m/s

b)what is its velocity before it reaches the ground

c) what is the magnitude of its acceleration at its maximum height?

## Homework Equations

vf=vi+at

xf=xi+vit+0.5at

vf^2=vi^2+2a(xf-xi)

## The Attempt at a Solution

I just want someone to tell me whether what I have done is right or wrong

so for a)

I took vi=20m/s

vf=0m/s (I was going to calculate how long it takes to reach the maximum peak)

then I used vf=vi+at (a as-9.8)

I got 2.04 s

Then I took vi=0

vf=40

then used the same thing and got t as ~4 and then I added them together

b) At first I calculated the distance it takes to go from the building to the peak

using xf=xi+vit+0.5at

xi=0

vi=20

t=2.04

a=-9.8

I got ~22m

Then I added to the 54

then I used vf^2=vi^2+2a(xf-xi)

vi=0

a=-9.8

xf=76

xi=0

and got vfinal

c) I said it was 9.8 m/s^2

So did I do anything wrong and if I did tell me what it is