# Knematics finding velocity

1. Nov 1, 2008

### davo

1. The problem statement, all variables and given/known data

A person throws a rock horizontally from the roof of a 50m building. The rock lands 70 m from the base of the building.
How long was the rock in the air in s?

I got 3.194. The online home work thingy (webassign) said I was right.

With what velocity was the rock thrown in m/s?

2. Relevant equations
I used the Xf equation and solved for time. Dont I just plug in the time i got in the last equation and actualy solve it? Well i tryed that but I just keep getting it wrong.

3. The attempt at a solution
70=-4.9(3.194)^2 + Vo(3.194) Is this right?

2. Nov 1, 2008

### djeitnstine

The way you need to set it up is imagine the rock is simply dropping because the x velocity doesn't matter. So s = vt + 1/2 at^2 s = 50m v = 0 a = 9.8m/s^2

3. Nov 1, 2008

### djeitnstine

50 = 0t + 1/2 9.8 t^2

4. Nov 1, 2008

### davo

what does s represent in this?

5. Nov 1, 2008

### davo

I think i did my algebra wrong because it said it was wrong.

(49.88+50)/3.194= 31.305

6. Nov 1, 2008

### djeitnstine

sorry s is displacement

7. Nov 1, 2008

### djeitnstine

50 = 0t + 1/2 9.8 t^2 this is the equation, the height 50 is equal to zero plus 1/2 9.8*t^2 (the rock is modeled as only dropping from 50 meters above the ground the x velocity has nothing to do with it)