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Homework Help: Finding displacement from PE and KE?

  1. Apr 3, 2012 #1
    1. The problem statement, all variables and given/known data
    We were assigned bookwork, we have a test on thursday, and I really don't understand the concepts of the lesson or what we are suppose to be doing. (My teacher is not the best by any standards.) Anyhow, here is the problem.

    " A 50.0 kg diver steps off a diving board and drops straight down into the water. The water provides an upward average net force of 1500 N. If the diver comes to rest 5.0 m below the water's surface, what is the total distance between the diving board and the diver's stopping point under water? "

    2. Relevant equations
    KE = 1/2mv^2
    PE = mgx

    3. The attempt at a solution
    I'm confused on the entire concept of Kinetic and Potential energy. In this problem it says I need to find the divers point underwater? The only thing I can think of is solving for Ek and then Ek equals PEg, and solve for displacement, but I dont have velocity, I asked my teacher for help, and he says those are the only equations we needed, even though later he said he hadn't even looked at the problem he'd assigned.. Any help is much appreciated, not only on this specific problem but concept wise as well.
  2. jcsd
  3. Apr 3, 2012 #2
    at the top of the board, the diver has PE = mgh
    at the bottom, he has KE = (1/2)*m*v^2 = mgh
    it takes a 1500 N force 5 meters to decelerate a 50 kg mass traveling at v
    so, you can find the initial velocity
    so you can find the KE
    set that equal to the PE, and solve for h
    add 5
  4. Apr 3, 2012 #3
    Oh yeah! I can do F=ma to get acceleration and plug that into V = ax +vi to get my velocity, and it says when the diver stops, so that's why you add 5 to the end to get 20.3 m . Thanks!
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