How High is the Cliff Based on Projectile Motion?

[majormuss]

Homework Statement

Projectile A is launched horizontally at a
speed of 20. meters per second from the top of
a cliff and strikes a level surface below, 3.0 seconds
later. Projectile B is launched horizontally
from the same location at a speed of 30. meters
per second.
Approximately how high is the cliff?
(1) 29 m (3) 60. m
(2) 44 m (4) 104 m

Homework Equations

d=tv

The Attempt at a Solution

On two occasions my answer turn out to be '3' and '4'. but the answer key says it's '2'. I have tried for hours but i can't find the right approach. Please offer me a good explanation of how I should work with this? or u can even give me a link to read more or something.

 

[Q_Goest]

hi majormuss. If the projectile is launched horizontally, the horizontal component of velocity doesn't change (except by air friction which is ignored). Only the verticle component of velocity changes due to the acceleration of gravity. In this particular case, the initial verticle velocity is zero because it was launched horizontally. The verticle velocity then should be identical to a rock dropped from the same point. Can you figure out how far a rock would fall in 3 seconds?