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Assuming a perfect transfer of energy

  1. Apr 9, 2014 #1
    Hi,

    I'm new here, hope I am posting this question in the right place.

    Ok, here is my question:
    Assuming a perfect transfer of stored energy into work, how long could a 9 volt battery lift 180 pounds (81.7kg)?

    Qualifier:
    By "perfect transfer", I mean to assume that there is zero resistance or loss of energy. Also assumed is some sort of machine that makes perfect use of energy, lifting the weight straight up when energy comes in and dropping it straight down as soon as the energy runs out.

    I can't even begin to think of how to calculate this!

    Thanks,
    MW
     
  2. jcsd
  3. Apr 9, 2014 #2

    Nugatory

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    Staff: Mentor

    You don't have enough information here.

    We need to know how much energy is stored in the battery (for batteries, this is often measured in amp-hours), and the voltage only tells us how that energy is delivered.

    We also need to know how quickly you want the load to rise. Given the right gear ratio, a hamster on a treadmill could lift that weight.

    Here's an example you might want to think about: The lead-acid battery in an automobile is a 12v battery. So is the battery in an (ancient) laptop computer. The automobile battery, connected to an electric winch, will easily pull a car weighing several tons up an incline, slowly. The laptop battery.... Nope.
     
  4. Apr 9, 2014 #3
    Oh, sorry!

    Ok, I'm picturing a little 9v battery, like one that might be used to power a hand held device. Attaching image.

    I have no idea how to calculate amp-hours, but I googled and found this: 500-800 mAh(milliamphours)

    Sound right?

    With regard to how quickly I "want the load to rise"... instantly. Can you go backwards? Instead of asking how long it takes to rise, ask how long before it falls?

    Imagine a hoverboard (think Back to the Future) powered by a 9v battery, with a 180 lbs man on it. How long could the battery keep the man up? Without something magical in play, like anti-gravity.
     

    Attached Files:

  5. Apr 9, 2014 #4
    This will depend on how much energy per second (power) is used by the specific device to keep the man up. If you knew this, you could look up some data sheets for the battery and see if the battery can provide this power and for how long.

    What you are asking is not a general principle, general physics question. To answer it you need to go into specifics of the devices.
     
  6. Apr 9, 2014 #5
  7. Apr 9, 2014 #6

    Nugatory

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    Staff: Mentor

    Yes, that's milliamp-hours. The total energy in the battery is the product of the voltage and the milliamp-hours.

    Sorry, still no answer... That's a science fiction device, not subject to the laws of physics, so the laws of physics won't tell us anything about how it might behave. Now, if you could find a spec sheet that tells us how much current it draws at what voltage to support this load, then we could do the calculation. Unfortunately the science fiction writing crowd tends to be a bit lax about publishing spec sheets :smile:

    However, there is calculation that we can do. The battery is good for 500 mAh at 9v, so it can deliver (power=current times voltage) 4.5 watts for an hour before it dies. 4.5 watts is enough power to lift a 100 kg person against the force of gravity at the less-than-exciting rate of 5 mm/second. That's assuming a discharge rate of 500 mA, which is pretty aggressive (and assuming that I didn't lose a few decimal places doing the math in my head).
     
  8. Apr 9, 2014 #7
    Sorry Nasu, didn't see your reply before my last post.

    And I apologize that what I am "asking is not a general principle, general physics question." Does that mean it cannot be discussed here?

    Nugatory... you totally busted me! :D

    I am indeed a writer, currently working through a final edit on a novel. The story is about a business student who stumbles into simple method for controlling gravity. He doesn't immediately understand the true significance of the discovery and ends up using it to create a hoverboard. When word gets out, chaos ensues.

    Anyway, there is a passage in my story where a scientist is watching a video the kid posted on youtube and...

    Maybe I should just post the passage? Will I get flamed out of here?

    I'm just looking for a simple number I can insert into the story in this passage, one that is somewhat realistic in what is, admittedly, a very unrealistic flight of fancy.
     
  9. Apr 9, 2014 #8
    Here is the passage (the focus of this is bolded out):

    Turning and placing his hand on the shoulder of a tall, lanky man in a tweed jacket, Letson continued. “This is Dr. Jacob Stevenson, professor emeritus at Caltech, still active with certain endeavors at JPL. He can answer your question about the NSA presence.”

    Stevenson wasn’t quite ready for his cue; he was too busy absorbing the antlike action taking place throughout the hangar. He cleared his throat. “Right. Yes. The National Security Agency got involved because of an anomaly reported by JSpOC. The report got sent up the chain at U.S. STRATCOM and was eventually handed over to the NSA for follow-up.”

    Letson recognized confusion on Giles’s face. “U.S. Strategic Command has a space control mission run by the Joint Functional Component Command for Space. JSpOC is the Joint Space Operations Center, a division of JFCC-Space. Alphabet soup, I know,” Letson said. “Basically JSpOC is a bunch of space garbage men.”

    “I’m not sure I’d put it that way,” replied Stevenson. The conversation had his full attention now. “JSpOC’s mission is to detect, track, and identify all man-made objects in Earth orbit. Because the Earth is constantly bombarded by meteors, certain algorithms are in place to screen out that activity as white noise. Ten days ago these algorithms kicked out a series of errors. Or at least what techs initially thought were errors.” Stevenson leaned in toward Giles to convey the excitement. “What they had actually detected were objects coming from Earth and burning up as they left the atmosphere!”

    Giles shrugged. “And the NSA tracked it to our target? Do they believe it’s a weapon?”

    Stevenson was flabbergasted at the obtuse nature of this response. “You don’t get it. The objects burned up in the atmosphere!”

    “Yes, sir. I heard you, sir,” Giles returned sharply. “That’s why you guys protect space shuttles with heat tiles, so they don’t burn up, right? Does the NSA think this kid is building missiles?”

    “Wrong! We protect shuttles for reentry!” Dr. Stevenson was animated now. “Outside of magnetic rail guns in a laboratory or ballistics requiring cutting-edge chemical propulsion under very controlled circumstances, there simply is no practical way to achieve the kind of velocities necessary to create enough friction to burn objects up while leaving the atmosphere! Certainly not at a nearly perfectly vertical trajectory! Yet JSpOC recorded six such events in the space of three minutes. This is the most amazing—”

    Letson interrupted, pulling the professor back by his shoulder. Stevenson was so excited that he was practically shouting in Giles’s face.

    “Jim, I brought Dr. Stevenson in here because I wanted him to impress upon you the significance of what we’re all taking part in here. Forgive his exuberance. You will understand in a moment. What’s important is that you realize that this is the single most important mission you have ever run for us.” As Letson released the doctor’s shoulder, he clarified. “In fact, obtaining this technology is the most important mission the Order has ever had.”

    Stevenson had been released, but now his lips moved as he struggled for the right words. Finally his shoulders relaxed and he gestured at a cluster of big screens. They were displaying an endless muted cycle of the videos that had been posted to YouTube under the username ZG_Enterprise, an account owned by Dale Adams.

    “Each one of these videos appears to be demonstrating a different phenomenon caused by manipulating a never-before-observed principle of physics. Your techs have analyzed the raw video exactly as it was uploaded—taken directly from YouTube’s servers—and have yet to find any sign of tampering or editing. Further,” he paused, pointing at the screen in the middle, which showed Dale Adams wearing large black earmuffs and laughing as he launched paint cans into oblivion, “the time stamp embedded in this particular video correlates closely to the timing of the anomalies recorded by JSpOC. Additionally, the NSA has followed the trajectory of the objects back to Dale Adams’s place of employment. It appears to be a near-certainty that this is not just an elaborate hoax, which was our initial take, as you might imagine.

    “Despite this evidence, I still would like to perform my own tests before jumping to any speculative conclusions. However, when Nathan came to me, he asked me to set aside any doubts, and just assume the videos are real. What he wanted to know was very simple: If gravity could truly be controlled as demonstrated in the videos, what were the implications?”

    Stevenson paused, wrestling with too many thoughts.

    “The most obvious benefit is an infinite source of force—a means of powering devices that create usable forms of energy,” he said, pointing to a screen that showed the guts of a shake light that had been pulled apart so the camera could witness a magnet traveling up and down through a coil to generate power. Eventually a little puff of dark smoke appeared, as a capacitor, overwhelmed by repeated energy surges, melted down. “This is a very crude device, but it does demonstrate that by controlling the force of gravity to drive a simple Faraday generator, energy can actually be created.

    "Now, I know what you're all thinking," Stevenson added quickly, casting about to see who was preparing to laugh at him. "Energy can neither be created nor destroyed, right? But if gravity can be controlled, it seems clear that there exist possibilities outside of Newton's Law of Conservation of Energy. This young man appears to be demonstrating a clear exception," Stevenson said, pointing to a screen that showed Dale Adams gliding around a parking lot on his hoverboard. "Assuming something as impossible as a perfect transfer of stored electricity into the force of work, we've calculated that a fully charged nine-volt battery should only be able to lift a one-hundred-eighty-pound mass, such as a man standing on a hoverboard, for eight point four seconds! [Calculate a real number for this]

    "Of course, e equals mc squared, so, theoretically, the work of lifting could be extended indefinitely if we allowed for the mass of the chemicals inside the battery to be consumed," Stevenson said, glancing around to see who was going down that path. "But the odds of a single person unlocking cold fusion and gravity at the same moment are laughable. It simply cannot be. No, it appears as though the battery isn’t being used to do the actual work of lifting; it’s merely sending a signal to tell a force of antigravity to repel the board away from the ground.”

    “Sir,” said a quiet female voice behind Giles. She was tapping his shoulder.

    “Not now,” Giles dismissed her gruffly.

    "The implication is this,” continued Stevenson. “That the forces of attraction and repulsion are not necessarily tied to a constant, in the unchangeable sense of the word. According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between them. The constant of proportionality, the big G, is the gravitational constant. But these videos seem to suggest that the big G can be unchained from the two masses and, instead of behaving as a constant, act more like a magnetic vector that relies on an input that can be tuned. It seems to suggest that the mass of the two objects can be made less relevant than the variant of a bond between them, something that can actually be controlled. This has fascinating implications to the mathematics of field theory because—"
     
  10. Apr 9, 2014 #9

    Nugatory

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    Staff: Mentor

    If you can restructure things a bit so that the power source is a small gasoline motor (lawnmower, weedeater, chainsaw, string trimmer) you'd be solidly inside the envelope of plausibility - these motors have way better power density than any battery.

    You don't see battery-powered aircraft on any large scale, because batteries just don't deliver enough power for long enough relative to their weight.
     
  11. Apr 9, 2014 #10

    jbriggs444

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    Science Advisor

    By "lift" you have subsequently clarified that you merely mean "support".

    Indefinitely. It takes no energy at all to continue supporting a man at a constant height.
     
  12. Apr 9, 2014 #11
    Thanks, Nugatory!

    Ok, here is a bit of restructuring:
    "Assuming something as impossible as a perfect transfer of stored electricity into a simple motor that could do the work of lifting––like a lawnmower engine––we've calculated that a fully charged nine-volt battery should only be able to lift a one-hundred-eighty-pound mass, such as a man standing on a hoverboard, for eight point four seconds!"

    How's that?
     
  13. Apr 9, 2014 #12
    Sorry, I coached the question wrong!
     
  14. Apr 9, 2014 #13
    No, I did not mean it cannot be discussed. Just that it is a sterile discussion if the details are not known.

    And I did not know that is about a SF scenario.
    I thought you are thinking about some sort of hovercraft machine.
    In SF you have a lot more freedom. :)
     
  15. Apr 9, 2014 #14
    I probably should have mentioned why I was asking in the OP, might have made it easier for you guys!

    :tongue:

    But thanks for the input!

    I'll keep my eye on this thread in case anybody has more thoughts.

    Since I'm already here, anybody want to parse this paragraph?
    "The implication is this,” continued Stevenson. “That the forces of attraction and repulsion are not necessarily tied to a constant, in the unchangeable sense of the word. According to the law of universal gravitation, the attractive force between two bodies is proportional to the product of their masses, and inversely proportional to the square of the distance between them. The constant of proportionality, the big G, is the gravitational constant. But these videos seem to suggest that the big G can be unchained from the two masses and, instead of behaving as a constant, act more like a magnetic vector that relies on an input that can be tuned. It seems to suggest that the mass of the two objects can be made less relevant than the variant of a bond between them, something that can actually be controlled. This has fascinating implications to the mathematics of field theory because—"

    Is that good enough for scifi or would it completely derail a reader who happened to have a strong grasp on physics, such as you guys?
     
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