Falling Rock Problem

  • Thread starter cybernerd
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  • #1
cybernerd
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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?
 

Answers and Replies

  • #2
rl.bhat
Homework Helper
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Can you find a kinematic equation which relates the initial velocity, final velocity, displacement and the acceleration?
 
  • #3
cybernerd
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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!
 
  • #4
rl.bhat
Homework Helper
4,433
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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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