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## Homework Statement

Two baseballs are set in motion from the top of a 5 m tall tower. One is thrown straight down toward the ground with an initial velocity of 20 m/s. The second is thrown straight up toward the sky with an initial velocity of 20 m/s.

How do their velocities compare when they hit the ground?

## Homework Equations

Vx^2=Vx0^2 + 2*a(x2-x1)

## The Attempt at a Solution

This what my teacher wrote:

#1 (Ball thrown straight down)

Vx0= 20 m/s

Vx=?

t=?

x=5 m

a=10 m/s^2

Vx^2=Vx0^2 + 2*a(x2-x1)

Vx^2=500

Vx = 22.36 m/s

#2 (Ball thrown straight up)

Vx0= -20 m/s

Vx= ?

a= 10 m/s^2

x= 5 m

Vx^2=Vx0^2 + 2*a(x2-x1)

Vx^2=(-20)^2 + 2(10)(5)

Vx^2=500

Vx=22.36 m/s

I don't understand why the initial velocity for the ball thrown straight up is negative. Also, since it's downward acceleration, wouldn't the acceleration for the ball thrown up be negative (it's going AGAINST gravity, correct?).