Solve 500g Falling Rock Problem: Initial Speed

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To solve the problem of a 500g rock thrown straight down from a bridge, the key is to use kinematic equations that relate initial velocity, final velocity, displacement, and acceleration. The relevant equation is vf^2 = vi^2 + 2ad, which can be rearranged to find the initial velocity (vi). The final velocity (vf) is given as 12.5 m/s, and the displacement (d) is 5.2 m. Additionally, applying Newton's second law can help determine the initial momentum and further clarify the initial speed. Understanding these concepts is crucial for solving the problem effectively.
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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?
 
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Can you find a kinematic equation which relates the initial velocity, final velocity, displacement and the acceleration?
 
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!
 
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.
 

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