Calculating Time for a Stone to Hit the Ground from a 100m Cliff

[Resmo112]

Homework Statement

30. A stone is thrown horizontally from a 100 m high cliff with an initial speed of 10.0 m/s. How long does it take to hit the ground?*

A.* 14.5 s

B.* 4.52 s

C.* 10.2 s

D.* 6.27 s
E. 19.2 s

Homework Equations

The 4 kinematics equations
DeltaV=a*t
DeltaX=vi*t+1/2a*T^2
Vf^2-Vi^2=2aDeltaX
DeltaX=1/2(Vf+vi)*t

The Attempt at a Solution

well first I tried to find the final velocity then use the change in velocity to get the time and I got 3.61s but the answer is 4.52 seconds, so I was told that since you're looking at a stone thrown horizontally so you're looking at a vector in 2 dimensions. so I set Vf to Zero where the velocity becomes 0 at the apex and i get 1.02 and obviously that's not the answer either. I'm really kind of stumped on this one and I don't see what else I can do. Any help is greatly appreciated.

 

[olivermsun]

Suppose the stone were simply dropped, would it take any longer to hit the ground?

 

[Resmo112]

Suppose the stone were simply dropped, would it take any longer to hit the ground?

NO! And I don't know why this was so hard, the X direction had a initial velocity of 10 m/s but the velocity in the y direction is still 0 so the answer is just squrrt2(100)/9.8 = t (reworked equation of X=1/2a*t2. THANK YOU SO MUcH THAT WAS DRIVING ME NUTS!