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Homework Help: I need help converting kg/s!

  1. Dec 8, 2004 #1
    Can someone help me make a connection between kg/s and kgxm^2/s^2?? It's a power problem...
    Water flows over a section of Niagra Falls at the rate of 1.2x10^6 kg/s and falls 50 m. How much power is generated by the falling water?
  2. jcsd
  3. Dec 8, 2004 #2
    Are you doing #36, because I just solved it, but I'm not sure if its correct. I solved the problem by making P=mgd/t
    Does that make sense?
    For my answer, I got 5.886*10^8 W, but again, I'm not sure if its correct, so Good Luck!
  4. Dec 9, 2004 #3


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    OK, so I was initially a bit lazy in trying to produce an explanation to this problem. However, I see a recurring problem here, and I think I should intervene to make sure this kind of bad habit does not continue. Most students at this level seems to be doing the same thing, and unless it is corrected, and corrected soon, the study of physics will become one mindless, mind-numbing process.

    First of all, when you look at a problem, ask yourself "What am I being tested on?" The problem at hand is trying to actually test if you understand the definition of the term "Power", as applied to mechanics. This is the concept that you are being tested on. You only need to understand the definition of the concept to be able to do this problem. You should NOT be memorizing any formulas which may or may not work for this particular case (that is a bad way to learn physics in the first place).

    So what is Power? Simply (and at this elementary level) it is defined as the rate of energy loss or gained. So it is the amount of energy per unit time that has been transfered. In SI units, it is the amount of energy in Joules that has been transfered per second. So the units of Power is Watts=Joules/s. So to answer this question, you need to know how much energy is being used up, or given off, in 1 second.

    Without having to give you anymore than you need, you should be able to figure out that, if you have a mass m, falling down a distance of y, then it has done a certain amount of gravitational potential energy. Even better, your problem tells you that this is the amount IN ONE SECOND, which is what we need. So this problem is practically solved by itself.

    Again, never, ever simply accept a concept in physics without understanding the definition. Always try to make sure you can "say" a concept or an equation in "words", because that is the easiest way to understand and comprehend it without having to memorize a bunch of disjointed information.

  5. Dec 9, 2004 #4


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    No offense,Zapper,i think the problem is a bit different.She doesn't realize what is given in the text and what fomulas and reasoning to use to get to what she needs...She"s given the flow and the distance and is asked for the power generated by the water's fall.
    U might be right,though,maybe the concepts are unclear and then she should proceed with revising them first,then attacking problems.

    Anyway,since the problem is simple and u already gave some hints,i'll state the answer:
    Power is approximately equal to 5 times 10^{8} Watts.
  6. Dec 9, 2004 #5
    Thanks everyone...I eventually figured it out and i must admit i felt kinda stupid for not knowing it...haha. Ok how did you know it was #36??? that's weird!!!
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