Solve 500g Falling Rock Problem: Initial Speed

[cybernerd]

Homework Statement

A 500g rock is thrown straight down from a bridge and hits the water 5.2 m below. If the rock strikes the water at a speed of 12.5 m/s, what was the initial speed of the rock?

Homework Equations

I'm not totally sure...

The Attempt at a Solution

I have no idea how to even start this. Could someone help me start to solve it, please?

 

[rl.bhat]

Can you find a kinematic equation which relates the initial velocity, final velocity, displacement and the acceleration?

 

[cybernerd]

It's apparently supposed to be a momentum equation...but I have to disagree with them there...

Well, I suppose vf^2 = vi^2 + 2ad is worth a shot.

Which gives me...

Vi^2 = vf^2 - 2ad

That works out to the right answer...but it doesn't look like it belongs on a momentum assignment.

Thanks!

 

[rl.bhat]

OK.
Find the final momentum and time to travel a distance 5.2 m. Force acting on the rock is known. Using Newton's second law of motion, find the initial momentum. From that you can find the initial velocity.