# Homework Help: Motion question

1. Sep 26, 2007

### jason.frost

1. A boy standing on the ground throws a stone with velocity 20 m/s at an angle of 30 degrees above the horizontal. It hits the wall of a nearby building at a position 3.0m above the level at which it was thrown. How far is the from the boy? (Two answers are possible. Give both. One represents the stone on its way up, the other on the way down.)

2. I guess I mainly need some help with which formula to use.

3. I am thinking that you will use Range=(Vo^2*sin2theta)/(|g|) and V^2x = V^2ox + 2axX. Sorry if those look bad. Is there any sub or sup script formatting? Am I thinking right with the formulas? I really don't even know what the second one finds for you.

2. Sep 26, 2007

### JoAuSc

You would use

$$y(t) = y_o + v_{yo} t - \frac {1}{2} g t^2$$

for vertical motion and

$$x(t) = x_0 + v_{xo} t$$
for horizontal motion. (Btw, see this thread for LaTex formatting.)

3. Sep 26, 2007

### jason.frost

Thanks.

What does the question mean by one answer is for "the way up and the other for the way down"?

4. Sep 26, 2007

### D H

Staff Emeritus
The stone could still be rising when it hits the wall ("on the way up") or it might have reached its maximum height prior to hitting the wall ("on the way down"). The two scenarios yield two different answers.