# Rock Drop Free Fall Problem

1. Dec 14, 2008

1. To determine how high a cliff is, a llama farmer drops a rock, and then 0.800 s later, throws another rock straight down at a velocity of −10.0 m/s. Both rocks land at the same time. How high is the cliff?

2. I know some kinematics equations must be used

3. I am stumped because I dont know the final velocity the height or the time just the difference in time between the two

2. Dec 14, 2008

### rl.bhat

What is your sign convention? Why the velocity is negative?

3. Dec 14, 2008

i dont know what you mean by sign convention but velocity is negative because it is traveling downwards

4. Dec 14, 2008

### rl.bhat

In that case, what is the direction of acceleration due to gravity, g?

5. Dec 14, 2008

-9.8

6. Dec 14, 2008

### fallen186

d=d0+v*t+1/2at2
d=d0+v*(t-.800s)+1/2a(t-.800s)2

First rock,
d0=0
V=0
a=9.81
d=1/2at2

Second rock,
d0=0
d=10m/s*(t-.800s)+1/2*9.81*(t-.800)2

I used calculator to find when these to intersect.
y1=1/2at2
y2=10m/s*(t-.800s)+1/2*9.81*(t-.800)2
Calc->Intersect
I got x =2.26sec y = 25.02 m