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Homework Help: Mechanical energy/efficiency problem

  1. May 1, 2010 #1
    1. The problem statement, all variables and given/known data

    (i)Water flows over a dam at the rate 580kg/s and falls vertically 88m before striking the turbine blades. Calculate the speed of the water just before striking the turbine blades (SOLVED) and (ii) the rate at which mechanical energy is transferred to the turbine blades assuming 55% efficiency.

    2. Relevant equations

    Ek = 1/2mv^2 Mechanical efficiency = Work out/Work in

    3. The attempt at a solution
    Answer to part (i) v=sqrt2gh

    v = 41.5 m/s

    Mechanical efficiency = work out/work in

    Work out = Ek1 - Ek2

    Work in = Ek1 - Ek2/55%

    Ok and I'm pretty much stuck. :(

    Thanks in advance for any help guys!
  2. jcsd
  3. May 1, 2010 #2
    This problem looks familiar did you post it on some other message board?

    As you stated the mechanical energy is the ratio of the work out to the work in. What's the work in? The efficiency is 55% so, 55% of the work in is returned.
  4. May 1, 2010 #3
    I'm having trouble understanding how to calculate the work in... :(

    Also it says calculate the "rate", so how would the answer be expressed? J/s?

    Edit: I've never posted this anywhere before.
  5. May 1, 2010 #4
    Work is Joules/s. 580 kg/s fall off the dam. So work in is the energy associated with the mass that falls each second. What's the energy associated with the mass?
  6. May 1, 2010 #5
    Kinetic energy? 1/2mv^2 = 1/2(580)(41.552)^2 = 500704 J = 500.704 KJ

    Is that the right calculation for the work in?
  7. May 1, 2010 #6
    Yes, now how does that relate to work in and finally, work out.
  8. May 1, 2010 #7
    Thanks, I understand it now!
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