# Homework Help: FreeFall Confusing Question?

1. Sep 4, 2008

### cmsoccer21

1. The problem statement, all variables and given/known data
A rock is thrown horizontally from a cliff with a speed of 15 m/s. It falls half of the heigh of the cliff in the last three seconds of its fall. What is the total fall time?

2. Relevant equations

The Kinematic Equations

3. The attempt at a solution
Well heres what I have so far but I dont know what its getting me.
Vi=Initial Velocity
Vo=Halfway Down Cliff Velocity
Vf=Final Velocity
Y=Height of cliff
t=time
Vf+29.4=-9.8t
Y=.5(0+Vo)t
Y=.5(Vo+Vf)3
Y=-4.9t^2
Y=Vo(-88.2)

These were all derived from kinematic eqautions.

2. Sep 4, 2008

### CompuChip

I'd (always) work from the general formula,
$$y = -\frac{1}{2} g t^2 + v_0 t + h_0$$.

You have one piece of information that you can plug into that formula, to replace the unknown height $h_0$ of the cliff by the falling time $t_f$. (You know $v_0$, but think before you write it down!) Then the remaining formula only contains the constant $g \approx 9,8$ and the falling time $t_f$ so you can set it to the final height (0) and solve for the latter.