Can I Assume Yi is 0m in Projectile Motion Calculations?

[EinsteinApple]

Homework Statement

A man builds a slingshot that can launch water balloons. If he launches a water balloon at 45.0 degrees to the horizontal at a speed of 35.0 m/s, how far away will the balloon hit?

Homework Equations

xf=xi + vix t
vfy=viy+at
yf=yi + 1/2(viy+Vfy)t
yf=yi + viyt + 1/2at^2
vfy² = viy²a + 2ay

The Attempt at a Solution

:

ti = 0
xi=0
yi = 0 (CAN I ASSUME THIS?)
viy = 24.75 m/s
vix = 24.75 m/s
ax=0m/s2
ay=0m/s2
[/B]

 

[kuruman]

As long as you are consistent with the interpretation of the other given quantities, yes you can.

 

[EinsteinApple]

I just assumed that y2 = 0 because it hits the ground. Therefore, I assumed because he was holding the slingshot y1 couldn't equal 0.
Therefore if y1=0, then y2= a negative value because it's lower.

 

[kuruman]

Is the balloon launched at some height above ground? What is that height?
When you write

yf=yi + 1/2(viy+Vfy)t

yi and yf need to be measured from the same point. That's what what I meant by "consistent".

 

[CWatters]

The problem statement says nothing about the launch or landing height so you have to assume they are the same.