7 ms·

I remember reading a comment somewhere on the web that made it "click" for me: "Gravity is so weak that force of a tiny fridge magnet is able to counter gravit

by bd 11y ago

I remember reading a comment somewhere on the web that made it "click" for me: "Gravity is so weak that force of a tiny fridge magnet is able to counter gravity pull of the whole planet".

tzs 11y ago
That approach has never really done it for me, because what makes it seem impressive is that the magnet is so much smaller than the planet. If you made the planet more dense, it would be smaller, making the comparison less impressive. Squeeze the planet all the way down to a black hole, and it would be about the size of the magnet. I think it works better to think of it in terms of the minimum force you can get. Imagine two test masses a fixed distance apart. The minimum non-zero electromagnetic force you can get between those two test masses is the force you get if each test mass has a charge equal to that of an electron. I'm not sure what the lowest mass particle is that you could reasonably hold near a test point, but since you can reasonably confine an electron to a small region the gravitational force between two electrons is an upper bound on the minimum gravitational force, and that is a ridiculous number of orders of magnitude lower than the minimum electromagnetic force.
- dhimes 11y ago
  The ratio of the electric force to the gravitational force for two protons is a good way to get a numerical feel, but the intuitive "aha!" is what the demos are trying to help with. There's a similar demo for showing the strength of atmospheric pressure. You can boil a little water in a soda can, and quickly invert it into a little bowl of water. The water vapor inside quickly condenses causing a startling implosion of the can. It gives an intuitive feel for what 1 atm actually is. You think of astronauts going to where they have to counteract that pressure, and what would happen on failure, and you think "holy shit." Then you think of James Cameron going to where the pressure is 1000 times as great, and what would happen on failure, and you think "holy fucking shit!!"
  - bd 11y ago
    > Then you think of James Cameron going to where the pressure is 1000 times as great, and what would happen on failure, and you think "holy fucking shit!!" Indeed, see Byford Dolphin diving bell accident (very NSFW/NSFL): https://en.wikipedia.org/wiki/Byford_Dolphin https://en.wikipedia.org/wiki/Byford_Dolphin
    - dhimes 11y ago
      Ugh. That's pretty horrible. Thanks for the link- I hadn't seen it.
- beachstartup 11y ago
  i think the point of the comparison is that the magnet is a few grams, and the earth is trillions upon trillions of kilograms.
- dogma1138 11y ago
  You need to squeeze the earth to a much smaller size than that of a fridge magnet to make it into a blackhole and even then you'll get a blackhole with a pull of 1g which would be like a cute puppy.
  - tzs 11y ago
    An Earth mass black hole would have a radius of about 9 mm. There are many refrigerator magnets around that size. The pull would be 1 g 6400 km away from it. At 1 m it would be over 10^13 g.
agumonkey 11y ago
IIRC a MIT professor says something similar. After rubbing a glass rod and lifting a piece of paper he asked is it a small or big force ? and then proceed to explain that the rod pulls more than a whole planet.
quesera 11y ago
Yes, but that tiny fridge magnet can't hold the moon in orbit around the earth, nor the earth around the sun. Perhaps if you replace "weak" with "diffuse"?
- ars 11y ago
  If the moon was entirely electrons, not only could it stay in orbit, it would have more energy than the entire visible universe! http://what-if.xkcd.com/140/ http://what-if.xkcd.com/140/ If your magnet was as weak as a fridge magnet and as large as the earth it would be way way way overpowered to hold the moon in orbit. So no, diffuse is wrong. Completely wrong. Weak is the correct word.
  - quesera 11y ago
    I realize the point is entirely semantic, but I think weak is also completely, equally wrong. Words are fun like that. If I stand accused of using the colloquial meaning of "weak" instead of the physics meaning, then OK. But a locomotive is strong. The pull of celestial bodies is stronger. The force of a locomotive is concentrated. The force of gravity is diffuse.
    - ars 11y ago
      The electromagnetic force, which is what we are comparing, is identically as "diffuse" as gravity. So using that word as a way to distinguish them is incorrect.
      - quesera 11y ago
        Conceptually and per unit measurement the force of gravity is many orders of magnitude weaker than the force of electromagnetism. No argument there. But that's just not the way the force of gravity is thought about by humans, because the gravitational effect is so small as to be undetectable between human-scaled bodies. So we attribute gravity to more massive things, and consider it only en masse, never divisibly. To the extent that the gravitational force of the earth has measurable effects on more things than the electromagnetic force of the refrigerator magnet, it is more...spread out. More distributed. More diffused. It follows the same inverse square law as everything else, but it does a lot more of it, over human-scaled distances. And this is how high school physics teachers can be counterintuitively correct, but I think the takeaway is more about how hopeless humans are at conceptualizing very large numbers than about physics.
DennisP 11y ago
Yes but on average, the mass of the planet is 4000 miles away. Put the magnet 4000 miles away and see how much pull you get.
- jerf 11y ago
  The magnet is also incredibly weak compared to what it could be. As it happens, the most recent "What If" from xkcd describes the difference in a more equal manner: http://what-if.xkcd.com/140/ http://what-if.xkcd.com/140/ Spoiler: "AAAAAAAAAAAAAAAAAAA!!!!!!"
- epistasis 11y ago
  That's a weird way to put it. The mass of the earth is ~6x10^27 grams, and the magnet is less than 10 grams.
  - DennisP 11y ago
    I'm not denying that gram for gram, gravity is much weaker.
- deleted 11y ago
  [deleted]
- light_hue_1 11y ago
  That's a confusing way to look at things. Lets make the two scenarios more similar. Say "earth" is a sphere with radius 1 meter and you're 1 meter away. And that puny magnet defeats its gravitational pull in the same way it defeats earth. How heavy would our sphere have to be in order to have the same pull as earth if it has a radius of 1 meter and our magnet is 1 meter away? If you remember Newton's law of gravitation the force an object will feel looks like: F=Gm/r^2. G is small, 10^-11 Nm^2/kg^2, m is the mass of earth 10^24 kg, r is normally 10^6 m (the radius of the earth). Plug in these numbers you get 10 m/s^2. The real answer is closer to 9.8, but we're looking for the order of magnitude here. We're still remarkably close for how much rounding we did. How lets say that we're now 1 meter away. How heavy would the mass have to be to still pull with 10m/s^2, which we know our magnet can defeat. (10^6)^2 ~> 10^12 so we have to be 10^24/10^12 ~> 10^12 kg. So to defeat a tiny magnet 1 meter away you need 10^12kg. Lets pretend our mass is a cube whose volume is 1m^3. That's 10^12kg/m^3. The center of the sun has a density of 10^5kg/m^3. This is 10^7 times denser! We're lucky that our sphere is far too light by many orders of magnitude to collapse into a black hole (maybe not so lucky because it's going to explode immediately!) but it's even denser than a white dwarf. This is on the order of the mass of Mt. Everest (this is a very rough and unprincipled comparison and when you unpack it can mean many different things, but it's something easy to visualize). So you can see. Gravity is indeed insanely weak. A magnet can defeat Mt. Everest.
analog31 11y ago
One difference between gravity and electromagnetism is that there are positive and negative charges in electromagnetism, that tend to cancel each other out over large distances. I don't think this is the thing that the theoreticians are looking for (no Nobel Prize for me), but it is a unique feature of gravity that gravity has only one polarity. And I don't remember how this goes with the strong and weak forces.
- mailarchis 11y ago
  This got me thinking about gravity's effect on antimatter.
  - analog31 11y ago
    Same gravitational "polarity" for both matter and antimatter. There is no repulsive gravitational force.
    - overgard 11y ago
      The inflation models of the universe actually depend on repulsive gravitational force, but it requires a very specific set of circumstances for it to happen.
    - nickhalfasleep 11y ago
      In theory, but I'm not sure if actual results have been published yet: http://arstechnica.com/science/2013/04/does-antimatter-fall-up-experiment-could-provide-the-answer/ http://arstechnica.com/science/2013/04/does-antimatter-fall-...
codezero 11y ago
The fridge magnet would not be able to do this without the surface friction of itself and the object it is attracted to.
- RealityVoid 11y ago
  Most likey, it would be able to do this. If you can place the fridge on a surface parallel to the ground, it should stay there (depends on the fridge magnet, but you get the gist)
  - codezero 11y ago
    The perpendicular force of the magnet to gravity doesn't negate gravity, the friction force does.
    - RealityVoid 11y ago
      That is correct, but only because of the particular position of the fridge magnet. The firction force from the magnet on the surface < magnetic pull(not always true, depends on the type of material the firdge and magnet is made of, but very likely true). Thus, if you put the magnet suspended on a horizontal surface, the magnetic force WOULD be the one holding it up.
- deleted 11y ago
  [deleted]
bryanrasmussen 11y ago
if a tiny piece of metal is on an orbit with an equal distance between earth and a tiny fridge magnet will the tiny fridge magnet really affect that metal more than the earth does? If I hold a tiny magnet 5 feet above the piece of metal, and the earth is 5 feet below the piece of metal. in which direction will the piece of metal move. In other words while I don't think my examples are any great ones, I don't think the example you're quoting really makes much sense either. On Preview: I see there's an explanation for why being far away from the magnet makes the magnet have less effect than being far away from a gravitational object https://news.ycombinator.com/item?id=10727977 https://news.ycombinator.com/item?id=10727977

by bd 11y ago

I remember reading a comment somewhere on the web that made it "click" for me: "Gravity is so weak that force of a tiny fridge magnet is able to counter gravity pull of the whole planet".

tzs 11y ago

That approach has never really done it for me, because what makes it seem impressive is that the magnet is so much smaller than the planet. If you made the planet more dense, it would be smaller, making the comparison less impressive. Squeeze the planet all the way down to a black hole, and it would be about the size of the magnet. I think it works better to think of it in terms of the minimum force you can get. Imagine two test masses a fixed distance apart. The minimum non-zero electromagnetic force you can get between those two test masses is the force you get if each test mass has a charge equal to that of an electron. I'm not sure what the lowest mass particle is that you could reasonably hold near a test point, but since you can reasonably confine an electron to a small region the gravitational force between two electrons is an upper bound on the minimum gravitational force, and that is a ridiculous number of orders of magnitude lower than the minimum electromagnetic force.

dhimes 11y ago
The ratio of the electric force to the gravitational force for two protons is a good way to get a numerical feel, but the intuitive "aha!" is what the demos are trying to help with. There's a similar demo for showing the strength of atmospheric pressure. You can boil a little water in a soda can, and quickly invert it into a little bowl of water. The water vapor inside quickly condenses causing a startling implosion of the can. It gives an intuitive feel for what 1 atm actually is. You think of astronauts going to where they have to counteract that pressure, and what would happen on failure, and you think "holy shit." Then you think of James Cameron going to where the pressure is 1000 times as great, and what would happen on failure, and you think "holy fucking shit!!"
- bd 11y ago
  > Then you think of James Cameron going to where the pressure is 1000 times as great, and what would happen on failure, and you think "holy fucking shit!!" Indeed, see Byford Dolphin diving bell accident (very NSFW/NSFL): https://en.wikipedia.org/wiki/Byford_Dolphin https://en.wikipedia.org/wiki/Byford_Dolphin
  - dhimes 11y ago
    Ugh. That's pretty horrible. Thanks for the link- I hadn't seen it.
beachstartup 11y ago
i think the point of the comparison is that the magnet is a few grams, and the earth is trillions upon trillions of kilograms.
dogma1138 11y ago
You need to squeeze the earth to a much smaller size than that of a fridge magnet to make it into a blackhole and even then you'll get a blackhole with a pull of 1g which would be like a cute puppy.
- tzs 11y ago
  An Earth mass black hole would have a radius of about 9 mm. There are many refrigerator magnets around that size. The pull would be 1 g 6400 km away from it. At 1 m it would be over 10^13 g.

agumonkey 11y ago

IIRC a MIT professor says something similar. After rubbing a glass rod and lifting a piece of paper he asked is it a small or big force ? and then proceed to explain that the rod pulls more than a whole planet.

quesera 11y ago

Yes, but that tiny fridge magnet can't hold the moon in orbit around the earth, nor the earth around the sun. Perhaps if you replace "weak" with "diffuse"?

ars 11y ago
If the moon was entirely electrons, not only could it stay in orbit, it would have more energy than the entire visible universe! http://what-if.xkcd.com/140/ http://what-if.xkcd.com/140/ If your magnet was as weak as a fridge magnet and as large as the earth it would be way way way overpowered to hold the moon in orbit. So no, diffuse is wrong. Completely wrong. Weak is the correct word.
- quesera 11y ago
  I realize the point is entirely semantic, but I think weak is also completely, equally wrong. Words are fun like that. If I stand accused of using the colloquial meaning of "weak" instead of the physics meaning, then OK. But a locomotive is strong. The pull of celestial bodies is stronger. The force of a locomotive is concentrated. The force of gravity is diffuse.
  - ars 11y ago
    The electromagnetic force, which is what we are comparing, is identically as "diffuse" as gravity. So using that word as a way to distinguish them is incorrect.
    - quesera 11y ago
      Conceptually and per unit measurement the force of gravity is many orders of magnitude weaker than the force of electromagnetism. No argument there. But that's just not the way the force of gravity is thought about by humans, because the gravitational effect is so small as to be undetectable between human-scaled bodies. So we attribute gravity to more massive things, and consider it only en masse, never divisibly. To the extent that the gravitational force of the earth has measurable effects on more things than the electromagnetic force of the refrigerator magnet, it is more...spread out. More distributed. More diffused. It follows the same inverse square law as everything else, but it does a lot more of it, over human-scaled distances. And this is how high school physics teachers can be counterintuitively correct, but I think the takeaway is more about how hopeless humans are at conceptualizing very large numbers than about physics.

DennisP 11y ago

Yes but on average, the mass of the planet is 4000 miles away. Put the magnet 4000 miles away and see how much pull you get.

jerf 11y ago
The magnet is also incredibly weak compared to what it could be. As it happens, the most recent "What If" from xkcd describes the difference in a more equal manner: http://what-if.xkcd.com/140/ http://what-if.xkcd.com/140/ Spoiler: "AAAAAAAAAAAAAAAAAAA!!!!!!"
epistasis 11y ago
That's a weird way to put it. The mass of the earth is ~6x10^27 grams, and the magnet is less than 10 grams.
- DennisP 11y ago
  I'm not denying that gram for gram, gravity is much weaker.
deleted 11y ago
[deleted]
light_hue_1 11y ago
That's a confusing way to look at things. Lets make the two scenarios more similar. Say "earth" is a sphere with radius 1 meter and you're 1 meter away. And that puny magnet defeats its gravitational pull in the same way it defeats earth. How heavy would our sphere have to be in order to have the same pull as earth if it has a radius of 1 meter and our magnet is 1 meter away? If you remember Newton's law of gravitation the force an object will feel looks like: F=Gm/r^2. G is small, 10^-11 Nm^2/kg^2, m is the mass of earth 10^24 kg, r is normally 10^6 m (the radius of the earth). Plug in these numbers you get 10 m/s^2. The real answer is closer to 9.8, but we're looking for the order of magnitude here. We're still remarkably close for how much rounding we did. How lets say that we're now 1 meter away. How heavy would the mass have to be to still pull with 10m/s^2, which we know our magnet can defeat. (10^6)^2 ~> 10^12 so we have to be 10^24/10^12 ~> 10^12 kg. So to defeat a tiny magnet 1 meter away you need 10^12kg. Lets pretend our mass is a cube whose volume is 1m^3. That's 10^12kg/m^3. The center of the sun has a density of 10^5kg/m^3. This is 10^7 times denser! We're lucky that our sphere is far too light by many orders of magnitude to collapse into a black hole (maybe not so lucky because it's going to explode immediately!) but it's even denser than a white dwarf. This is on the order of the mass of Mt. Everest (this is a very rough and unprincipled comparison and when you unpack it can mean many different things, but it's something easy to visualize). So you can see. Gravity is indeed insanely weak. A magnet can defeat Mt. Everest.

analog31 11y ago

One difference between gravity and electromagnetism is that there are positive and negative charges in electromagnetism, that tend to cancel each other out over large distances. I don't think this is the thing that the theoreticians are looking for (no Nobel Prize for me), but it is a unique feature of gravity that gravity has only one polarity. And I don't remember how this goes with the strong and weak forces.

mailarchis 11y ago
This got me thinking about gravity's effect on antimatter.
- analog31 11y ago
  Same gravitational "polarity" for both matter and antimatter. There is no repulsive gravitational force.
  - overgard 11y ago
    The inflation models of the universe actually depend on repulsive gravitational force, but it requires a very specific set of circumstances for it to happen.
  - nickhalfasleep 11y ago
    In theory, but I'm not sure if actual results have been published yet: http://arstechnica.com/science/2013/04/does-antimatter-fall-up-experiment-could-provide-the-answer/ http://arstechnica.com/science/2013/04/does-antimatter-fall-...

codezero 11y ago

The fridge magnet would not be able to do this without the surface friction of itself and the object it is attracted to.

RealityVoid 11y ago
Most likey, it would be able to do this. If you can place the fridge on a surface parallel to the ground, it should stay there (depends on the fridge magnet, but you get the gist)
- codezero 11y ago
  The perpendicular force of the magnet to gravity doesn't negate gravity, the friction force does.
  - RealityVoid 11y ago
    That is correct, but only because of the particular position of the fridge magnet. The firction force from the magnet on the surface < magnetic pull(not always true, depends on the type of material the firdge and magnet is made of, but very likely true). Thus, if you put the magnet suspended on a horizontal surface, the magnetic force WOULD be the one holding it up.
deleted 11y ago
[deleted]

bryanrasmussen 11y ago

if a tiny piece of metal is on an orbit with an equal distance between earth and a tiny fridge magnet will the tiny fridge magnet really affect that metal more than the earth does? If I hold a tiny magnet 5 feet above the piece of metal, and the earth is 5 feet below the piece of metal. in which direction will the piece of metal move. In other words while I don't think my examples are any great ones, I don't think the example you're quoting really makes much sense either. On Preview: I see there's an explanation for why being far away from the magnet makes the magnet have less effect than being far away from a gravitational object https://news.ycombinator.com/item?id=10727977 https://news.ycombinator.com/item?id=10727977