# Homework Help: Initial Velocity (Without time and acceleration)

1. Sep 16, 2009

### djtsmith007

1. The problem statement, all variables and given/known data
You are standing at the top of a cliff that has a stair step configuration. There is a vertical drop of 7 m at your feet, then a horizontal shelf of 9 m, then another drop of 3 m to the bottom of the canyon, which has a horizontal floor. You kick a 0.26 kg rock, giving it an initial horizontal velocity that barely clears the shelf below.

What initial horizontal velocity v will be required to barely clear the edge of the shelf
below you? How far from the bottom of the second cliff with the rock land?

2. Relevant equations

d=Vi(t)+(1/2)(a)(t^2)

3. The attempt at a solution
Cant seem to get around the problem without knowing more.

Last edited: Sep 16, 2009
2. Sep 16, 2009

### kuruman

Can you find the time required for the rock to reach the point where it barely clears the edge of the shelf?

Last edited: Sep 16, 2009
3. Sep 16, 2009

### djtsmith007

I havent been able to

4. Sep 16, 2009

### Anden

You can find the time by using gravity and the fact that the first step is 7 meters down. How long does it take for the rock to fall 7 m? If you find it you can calculate how fast it must travel in a horizontal direction.

5. Sep 16, 2009

### djtsmith007

Could you help me out a little more?

6. Sep 16, 2009

### djtsmith007

So im assuming you mean:
7m/9.8=.7142857s to make it to the bottom =t

using

d=(Vi+Vf/2)(t)
therefore
9m=(0+Vf/2)(.7142857s)
Vf=25.2

that not right?

7. Sep 16, 2009

### djtsmith007

anyone?

8. Sep 16, 2009

### kuruman

Use the equation that you initially quoted as "relevant" for the vertical motion. Identify ll the quantities - you know all of them except the time. Remember, vertical motion only.