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Falling Rock Problem

  1. Mar 1, 2009 #1
    1. The problem statement, all variables and given/known data

    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?

    2. Relevant equations

    I'm not totally sure...

    3. The attempt at a solution

    I have no idea how to even start this. Could someone help me start to solve it, please?
     
  2. jcsd
  3. Mar 2, 2009 #2

    rl.bhat

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    Homework Helper

    Can you find a kinematic equation which relates the initial velocity, final velocity, displacement and the acceleration?
     
  4. Mar 2, 2009 #3
    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!
     
  5. Mar 2, 2009 #4

    rl.bhat

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    Homework Helper

    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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