4 ms·

Complete aside: Has anyone actually implemented this classical example? I've heard it mentioned a few times but always assumed the people floating it hadn't th

by dguest 5y ago

Complete aside: Has anyone actually implemented this classical example? I've heard it mentioned a few times but always assumed the people floating it hadn't thought about orders of magnitude enough: a "sip" of orange juice is indeed a lot of energy, about 80 kJ [1], and if you could extract that much energy from one person every 10 seconds you'd get 8 kW, which could power a decent sized lobby if the lighting is efficient. But to extract 80 kJ over 1 m... you have to push with 80 kN, or 17 thousand pounds (or 1.4 TRex bites worth of force [2]). More realistically, if we have everyone push with an annoying but manageable 50 N (about 10 pounds), we'd get 5 W, which might power a single (dim) LED bulb. As I said, this is a bit off topic for the original post, but the common theme is that people tend to confuse "extracting some energy" with "extracting a useful amount of energy in a cost effective way" (they also tend to overestimate mechanical energy). This on top of the thermodynamic constraints you mention to start with. [1]: https://www.wolframalpha.com/input/?i=energy+in+40+grams+of+orange+juice https://www.wolframalpha.com/input/?i=energy+in+40+grams+of+... [2]: https://www.wolframalpha.com/input/?i=80+kN https://www.wolframalpha.com/input/?i=80+kN

bryanrasmussen 5y ago
I don't know if anyone ever implemented the classical example, I just see it floated every few years.
- dguest 5y ago
  I see it every few years as well, and I'm glad I finally did the math (it's not that hard actually)! It's probably a good freshman physics example, to make people think about orders of magnitude and energy. Or a good "Fermi question" if you're one of those places that still interviews with those.
  - addaon 5y ago
    A shortcut for this is that a horsepower is about 750 watts (745.7 or so), and that you can extract one horsepower from a fit human for a very limited amount of time before they stop humaning. You can sustain about half this, or a little less, with the correct human. These super-approximate numbers come in useful for, e.g., human-powered airplane design. (The equivalent horsepower numbers for a horse are about 15 hp peak, 2 hp sustained. Naming is hard.) So, if you're proposing a system that extracts energy from N humans at a time, and you're hoping to get more than N hp out of it... you're probably off by a bit. If you're hoping to do this without the humans noticing, you're probably off by an order of magnitude.
beagle3 5y ago
I’ve heard the story (myth?) that Edison’s house had a gate that was relatively hard to push, and which visitors asked him to fix but he refused - and it eventually turned out it was providing energy to his locally dug well, pulling some water out with every opening @ closing of the gate. Might be an urban legend.
- SketchySeaBeast 5y ago
  Based upon this[1] (so big old grain of sea salt required) it's an urban myth. [1] https://boards.straightdope.com/t/thomas-edisons-front-gate/120679/6 https://boards.straightdope.com/t/thomas-edisons-front-gate/... I also wonder at the practicality of pulling out one sporadic push worth of water out of a well. No way is that moving much water.
MrDunham 5y ago
I vaguely recall back in one of my engineering classes we did the math on turning all of the stationary bikes at the gym into power generating units. I believe the result was that the value of power generated was so utterly dwarfed by the costs in copper, inverters, etc. - not to mention additional maintenance costs - that it was at least an order of magnitude outside of making sense. Some of those economics certainly have changed over the last 15 years but many (I’m looking at you, copper) are rather fixed and won’t go down over time. Also > 1.4 TRex bites worth of force I have never seen T Rex bites as a unit of measurement before and I love it.
- codethief 5y ago
  > I vaguely recall back in one of my engineering classes we did the math on turning all of the stationary bikes at the gym into power generating units. Very much related: https://www.youtube.com/watch?v=S4O5voOCqAQ https://www.youtube.com/watch?v=S4O5voOCqAQ
jerf 5y ago
Some of them have been implemented at small scale. I believe the "floor that turns foot traffic into power" has been implemented a couple of times. Certainly the "lift a rock to power a light" has been implemented several times. I believe someone even stuffed a soccer ball or three with things that generate power when the ball is kicked. In fact it's a perennial design winner in various design contests that are judged by designers and not engineers or scientists. It looks awesome when you design some cool little gadget that "harvests" enough energy to power a small town from a soccer ball or something. The fact that it doesn't work is not a disqualifier in those contests. It't totally what I'd enter at this point if I were going to enter such a contest, it's an obviously winning play. But the floor that converts foot traffic to energy doesn't generate much energy, and is very unpleasant to walk on. The soccer ball that extracts energy from the kick means that it has extracted energy from the kick, so it doesn't go flying away like a soccer ball should but is more likely playing with a ball of damp laundry. Guess what kids don't want to play with? The "lift a rock to power a light" does work, but it provides a lot less light for a lot less time than people think, and the rock ends up being pretty heavy. (But it does have at least some legitimate use cases, just not as many as you might hope.) The bike that extracts energy from the biker is so insanely unmarketable that what's winning in the market right now is the exact opposite, the electric bikes that supplement the biker's power. (Also I will add as honorable mention the "solar powered X" that is spec'ed with one or two orders of magnitude too few solar panels. Putting panels on the top of your electric car may even be theoretically worthwhile in the long run, but only as a small supplement to the car. The idea that you'll be toodling down the highway at 75mph on 10ish square feet of solar panels is risible. Multiply that by some large two-digit numbers.)
jhgb 5y ago
> a "sip" of orange juice is indeed a lot of energy, about 80 kJ [1], and if you could extract that much energy from one person every 10 seconds you'd get 8 kW, which could power a decent sized lobby if the lighting is efficient. What's the total energy expended to produce those 80 kJ worth of orange juice, though? You need to power agricultural machines, food trucks, etc. etc.

by dguest 5y ago

bryanrasmussen 5y ago

I don't know if anyone ever implemented the classical example, I just see it floated every few years.

dguest 5y ago
I see it every few years as well, and I'm glad I finally did the math (it's not that hard actually)! It's probably a good freshman physics example, to make people think about orders of magnitude and energy. Or a good "Fermi question" if you're one of those places that still interviews with those.
- addaon 5y ago
  A shortcut for this is that a horsepower is about 750 watts (745.7 or so), and that you can extract one horsepower from a fit human for a very limited amount of time before they stop humaning. You can sustain about half this, or a little less, with the correct human. These super-approximate numbers come in useful for, e.g., human-powered airplane design. (The equivalent horsepower numbers for a horse are about 15 hp peak, 2 hp sustained. Naming is hard.) So, if you're proposing a system that extracts energy from N humans at a time, and you're hoping to get more than N hp out of it... you're probably off by a bit. If you're hoping to do this without the humans noticing, you're probably off by an order of magnitude.

beagle3 5y ago

I’ve heard the story (myth?) that Edison’s house had a gate that was relatively hard to push, and which visitors asked him to fix but he refused - and it eventually turned out it was providing energy to his locally dug well, pulling some water out with every opening @ closing of the gate. Might be an urban legend.

SketchySeaBeast 5y ago
Based upon this[1] (so big old grain of sea salt required) it's an urban myth. [1] https://boards.straightdope.com/t/thomas-edisons-front-gate/120679/6 https://boards.straightdope.com/t/thomas-edisons-front-gate/... I also wonder at the practicality of pulling out one sporadic push worth of water out of a well. No way is that moving much water.

MrDunham 5y ago

I vaguely recall back in one of my engineering classes we did the math on turning all of the stationary bikes at the gym into power generating units. I believe the result was that the value of power generated was so utterly dwarfed by the costs in copper, inverters, etc. - not to mention additional maintenance costs - that it was at least an order of magnitude outside of making sense. Some of those economics certainly have changed over the last 15 years but many (I’m looking at you, copper) are rather fixed and won’t go down over time. Also > 1.4 TRex bites worth of force I have never seen T Rex bites as a unit of measurement before and I love it.

codethief 5y ago
> I vaguely recall back in one of my engineering classes we did the math on turning all of the stationary bikes at the gym into power generating units. Very much related: https://www.youtube.com/watch?v=S4O5voOCqAQ https://www.youtube.com/watch?v=S4O5voOCqAQ

jerf 5y ago

Some of them have been implemented at small scale. I believe the "floor that turns foot traffic into power" has been implemented a couple of times. Certainly the "lift a rock to power a light" has been implemented several times. I believe someone even stuffed a soccer ball or three with things that generate power when the ball is kicked. In fact it's a perennial design winner in various design contests that are judged by designers and not engineers or scientists. It looks awesome when you design some cool little gadget that "harvests" enough energy to power a small town from a soccer ball or something. The fact that it doesn't work is not a disqualifier in those contests. It't totally what I'd enter at this point if I were going to enter such a contest, it's an obviously winning play. But the floor that converts foot traffic to energy doesn't generate much energy, and is very unpleasant to walk on. The soccer ball that extracts energy from the kick means that it has extracted energy from the kick, so it doesn't go flying away like a soccer ball should but is more likely playing with a ball of damp laundry. Guess what kids don't want to play with? The "lift a rock to power a light" does work, but it provides a lot less light for a lot less time than people think, and the rock ends up being pretty heavy. (But it does have at least some legitimate use cases, just not as many as you might hope.) The bike that extracts energy from the biker is so insanely unmarketable that what's winning in the market right now is the exact opposite, the electric bikes that supplement the biker's power. (Also I will add as honorable mention the "solar powered X" that is spec'ed with one or two orders of magnitude too few solar panels. Putting panels on the top of your electric car may even be theoretically worthwhile in the long run, but only as a small supplement to the car. The idea that you'll be toodling down the highway at 75mph on 10ish square feet of solar panels is risible. Multiply that by some large two-digit numbers.)

jhgb 5y ago

> a "sip" of orange juice is indeed a lot of energy, about 80 kJ [1], and if you could extract that much energy from one person every 10 seconds you'd get 8 kW, which could power a decent sized lobby if the lighting is efficient. What's the total energy expended to produce those 80 kJ worth of orange juice, though? You need to power agricultural machines, food trucks, etc. etc.