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Relating Newtons to Watt-hrs

  1. Jul 9, 2004 #1

    I'm working on a mythical gadget for an sf novel that converts electrical power into acceleration. I'm having a hard time figuring out this relationship from my science references. I suspect I've forgot some calculus that might be required.

    I'd like to figure out the real relationship between Netwons and Watt-hrs of electricity so I can make a statement like "at 100% efficiency, the gadget would consume x Watt-hrs of electric power and produce y Netwons of force each hour."

    Can someone give me a formula?
  2. jcsd
  3. Jul 9, 2004 #2

    Chi Meson

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    You have a classic case of "how many apples in an orange" here:

    the "newton" is a unit of force and the watt-hour is a unit of energy. If you want the unit of power, then you are talking about just simply "watt."

    Push a box with 100 newtons of force (that's about 20 pounds of force) across the floor for a distance of 20 meters and you have just used 2000 joules of your own energy. If you do this in 10 seconds then your power output was 200 watts. If you did it in 5 seconds then your power was 400 watts.

    Power is "transfer of energy" divided by time. Energy transfer is force times distance. So power is force times distance divided by time.

    And nothing will be 100% efficient; 98% efficiency would itself be quite Sci-Fi.
  4. Jul 9, 2004 #3


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    The output of a generator is energy, not force. Try to think out your idea so that it makes more sense. For conversion purposes, 1 joule = 1 watt-second. Also 1 joule = 1 newton-meter.
  5. Jul 10, 2004 #4
    Relating Watt-hrs to Newtons


    I'm developing a mythical "reactionless drive" for an SF novel. I'd like to be able to say something like "With 50% efficiency, the unit produces x Newtons of force for one hour for every y Watt-hrs of electricity input.

    I'm having difficulty figuring out my old college texts. Is there a formula that would convert Newton-hrs to Watts-hrs?

    - Alan
  6. Jul 10, 2004 #5

    Doc Al

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    don't double post

    Please don't post the same question in multiple forums. You've already asked this question here: https://www.physicsforums.com/showthread.php?t=34417
  7. Jul 10, 2004 #6


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    Hello alan,

    From a scientific standpoint it looks like your question posted in General Physics was competently answered by Chi Meson and Mathman within two hours of your posting it
    yesterday around 4PM.

    Your posting it again this morning in General Engineering forum makes me think you have some other question in mind----maybe a literary one----that you havent yet managed to get across to the assembled PF experts :smile:

    maybe it has to do with words
    no one on the other thread mentioned the beautiful and authentic word "impulse"
    A newton-hour is a quantity of impulse
    a product of thrust and time------force x time.

    the conventional SI metric unit of impulse is the "newton-second"

    From your post I guess maybe you are writing an SF story and you want to have people talk in a technical-sounding way about a mythical thrustor

    which when you feed it some energy----like a Megajoule (unless you prefer to say watt-hours)-----produces some quantity of impulse----like a newton-second.

    You want some knowledgeable-sounding engineer-type in your story to say, like, "this is the latest reactionless drive! it is very efficient!
    we get 1.2 newton-seconds of impulse per Megajoule!"

    Actually it would perhaps sound less dorky to say that we get
    1.2 newtons of thrust per Megawatt of power.

    1.2 newtons of thrust per megawatt sounds nice and mythical

    try it in the story and see :smile:
    Last edited: Jul 10, 2004
  8. Jul 10, 2004 #7


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    alan, another thing,

    you mentioned "50 percent efficiency"

    this is meaningful if you compare, say, electric energy in and kinetic energy out

    Maybe putting in one Megajoule of electricity
    will give the spaceship one half Megajoule of kinetic energy
    relative to a mythical absolute space reference frame called the

    mythical things always sound a bit crazy----the important thing is to
    try to make them not sound dorky (crazy is OK but crazy AND dorky is too much)

    are you actually writing the novel or are you helping someone else?
  9. Jul 11, 2004 #8
    Hello and thanks for the replies!

    My college physics and calculus is 20 years in the past, so I'm rusty.

    Doc: Eight hours after I made my original post, I was unable to find it, so I posted again. I assumed the original post had failed, either because I forgot to hit "submit" after previewing, or some glitch in initializing my account here. What is the standard for moderation on this board? Do you prefer replies by private message or public response?

    Marcus: Actually, my novel is complete. I'm doing final polish. No publisher yet, but I've sold short stories in the past and I figure I have a decent chance with this novel. To be honest, the reactionless drive is only a bit of background, and numbers only get mentioned once in the whole book, but I'd still like to have an authentic foundation for those numbers.

    Do you know Larry Niven's term "balonium?" It's a substance or technology that justifies a science fiction gadget. When he uses it, it usually has many real-world engineering constraints: heat loss due to inefficiency, power requirements and limits.

    My balonium is an integrated circuit chip, with millions of solid state quantum interferance devices that convert electrical input into kinetic energy. It's called Direct Kinetic Conversion (DKC). The technology has existed for several millenia, so it's well-developed. It's used in stacks to drive personal flight packs, vehicles, game balls, and space ships - and is even safe enough to implant in the human skeleton so individuals can fly (where the power comes from in this case is another bunch of balonium).

    The 50% efficiency is a comparison of electric energy in verses kinetic energy out. The 50% efficiency is just an example. I know that every real-world application loses energy in the process of working, so I thought I'd find out the ideal 100%, then twiddle the dial down to reflect loss. (I suppose loss would be heat, radiation, and even a certain percentage of thrust directed in the wrong direction.) At some point, I'd also like to find out some real world efficiency ratios for various kinds of engines: a rocket, an electric motor, etc. Any suggestions where I might find numbers like that?

    Thanks for your time!

    - Alan
  10. Jul 11, 2004 #9

    Doc Al

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    It's a PF no no to post the same message in multiple forums. But don't worry about it. :smile:
    What do you mean?
  11. Jul 11, 2004 #10

    My college physics and calculus is 20 years in the past, so I'm rusty. I'm aware that Watt-hours and Newtons are different beasts. I did derive the newton-meter equivalent for watt-seconds but it didn't seem to meet my purpose.

    I have a science fiction device that converts electricity into kinetic energy. I want to say "to accelerate x metric tonnes at 10m/[tex]sec^2[/tex] for 1 second requires y amount of electricity."

    What do I have to add to this statement so I can express the required electricity input in watt-hrs?
    Last edited: Jul 11, 2004
  12. Jul 11, 2004 #11


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    for simplicity suppose it is not X tonnes but one tonne

    and you accelerate for one second so the speed is 10 m/s

    there is this kinetic energy formula (1/2)M v2

    after acceleration the 1000 kg mass has K.E. of

    50,000 joules

    a joule of electricity is a watt-second (same thing by definition)


    so 50,000 watt-seconds of electicity is the same amount of energy
    (namely 50,000 joules) as the K.E. of a tonne mass traveling 10 m/s

    for two tonnes, you would have to double it (you learned about such proportionality in physics)


    the conventional force that would have been needed to do that acceleration
    is 10,000 newtons

    (this would accelerate 1000 kg by 10 m/s in one second)

    so a power load of 50,000 watts, assuming perfect efficiency in being converted to K.E., has the same effect as a force of 10,000 newtons.


    just thinking these things would set off a dozen or so car alarms in
    the physics faculty parking lot, so be sure you are safely clear of any
    academic institutions

    ultimately if one is going to have a conversion between KE and momentum one may be forced to imagine an absolute reference frame for the universe, a kind of background idea of stationarity (like "stationary with respect to the expansion of the universe" or "stationary with respect to the cosmic microwave background")------however at this point I jump ship and you need a new balonium expert: someone a good deal more creative
    Last edited: Jul 11, 2004
  13. Jul 14, 2004 #12
    Thanks Marcus et al,

    That gives me what I need.

    - Alan
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