Calculating Velocity and Time of a Horizontal Projectile

[davo]

Homework Statement

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?

Homework 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.

The Attempt at a Solution

70=-4.9(3.194)^2 + Vo(3.194) Is this right?

 

[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

 

[djeitnstine]

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

 

[davo]

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

what does s represent in this?

 

[davo]

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

(49.88+50)/3.194= 31.305

 

[djeitnstine]

sorry s is displacement

 

[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)