5 ms·

this doesn't really comapre the two forces qualitatively I think.. Suppose you did the same experiment on Jupiter; you would fall a lot faster but the speed of

this doesn't really comapre the two forces qualitatively I think.. Suppose you did the same experiment on Jupiter; you would fall a lot faster but the speed of decelleration wouldn't be affected. Perhaps on something like a neutron star the two times would even be similar, leading you to conclude the two forces are similar in magnitude... Incidentally, it's for a large part statistical mechanics, not electromagnetism which would halt you fall - the hardness of solid material is due in a significant part to the electrons not wanting to share orbitals, and not just to them repelling each other.

PieSquared 17y ago
And why do electrons not want to share orbitals?
- graphene 17y ago
  Because that's just the way they are :-) More precisely, no two electrons can ever occupy exactly the same quantum mechanical state. http://en.wikipedia.org/wiki/Pauli_exclusion http://en.wikipedia.org/wiki/Pauli_exclusion
  - scotty79 17y ago
    Why the thing that keeps them from occupying the same quantum mechanical state by holding them apart (like in neutron stars) is not called force (the fifth one)? That's actually quite funny that we call 'fundamental forces' only the forces that we can't explain the source of.
- deleted 17y ago
  [deleted]
- btilly 17y ago
  Because they are fermions, not bosons. Which has to do with their spin, not their charge. See http://en.wikipedia.org/wiki/Pauli_exclusion_principle http://en.wikipedia.org/wiki/Pauli_exclusion_principle for more.
btilly 17y ago
Obviously the structure of an atom is affected by electrons not wanting to share orbitals. But isn't it electromagnetism that keeps atoms from passing through each other? And isn't it also electromagnetism that causes groups of atoms to clump in molecules, then the molecules to stick together in a crystalline pattern? (Update, the answers respectively seem to be "not entirely" and "yes".)
- graphene 17y ago
  If that were true, why doesn't all matter stick together in one giant gob? If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference? In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible.
  - btilly 17y ago
    If that were true, why doesn't all matter stick together in one giant gob? A typical atom has a net charge of 0 with minor local variations. Therefore its attractive force drops off much faster than 1/r^2. And so electromagnetic attraction between atoms effectively disappears at any significant distance. If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference? It is possible for both rigid and non-rigid materials to satisfy the Pauli exclusion principle. Therefore the rigidity of things around us cannot be caused by the Pauli exclusion principle. In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible. Are you talking about volume or hardness? You are right that the exclusion principle results in volume. See http://en.wikipedia.org/wiki/Pauli_exclusion_principle#Stability_of_matter http://en.wikipedia.org/wiki/Pauli_exclusion_principle#Stabi... for verification. (That effect is much bigger than I had realized.) However it does not necessarily result in rigid things that are hard. Consider, for example, dropping a bucket of fresh water into salt water. Does the fresh water stop quickly upon impact? Obviously not. Now consider dropping a rock in the dirt. Does the rock stop quickly upon impact? Obviously it does. What is different between the two scenarios? The answer is the structure of the materials, which are held together by electromagnetic forces. Therefore it really is electromagnetic forces that quickly stop falling solid objects that hit solid things.
RiderOfGiraffes 17y ago
The Pauli exclusion principle pertains, does it not, to electrons (etc) not wanting to duplicate QM states, but is it not still the case that one atom not passing through another atom is becuase of the repulsion of the orbitals? Electrons in a single atom is a different question from electrons in different atoms. Of course, I am probably wrong, and look forward to your pointers to further reading to help me understand more.
- lucifer 17y ago
  Not just electrons. Take a look at Fermions: http://en.wikipedia.org/wiki/Fermion http://en.wikipedia.org/wiki/Fermion
- slackenerny 17y ago
  Most detailed explanation in the blogosphere is "Why Stuff is Hard" by Jim Pivarski, http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-hard/ http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-har... http://cornellmath.wordpress.com/2007/08/11/why-everything-is-hard/ http://cornellmath.wordpress.com/2007/08/11/why-everything-i... http://cornellmath.wordpress.com/2007/08/11/why-everything-is-hard/#comment-253 http://cornellmath.wordpress.com/2007/08/11/why-everything-i... Complete argument is the Dyson, Lieb and Thirring's proof of stability of matter, http://projecteuclid.org/euclid.bams/1183555452 http://projecteuclid.org/euclid.bams/1183555452 .
  - anatoly 17y ago
    Many thanks for that last link.
- graphene 17y ago
  But if one atom "tries" to pass through another, the electrons from both atoms will be spatially very close to each other, and consequently, they will be occupying very similar states. Coulombic repulsion certainly also plays a role, but it's not the only force at work here. I don't know the relative magnitudes for single atoms bumping into each other, but for bulk material, I believe it's about 40% exclusion, 60% coulomb. Why would electrons in different atoms interact differently than electrons in the same atom? They don't, as far as I know. Furthermore, if electrons from different atoms only interacted Coulombically (repelling each other), molecules and solids would never get formed. These are difficult questions, not addressed in the clear-cut cases often presented in introductory QM textbooks. I can't think of any must-read references right now, but if you're interested in this kind of thing I know of two books on molecular quantum mechanics; one by McWeeney (methods of molecular quantum mechanics) and one by Atkins (molecular quantum mechanics), which I haven't read myself, but I hear it's good. I don't know of any free online sources for this kind of stuff, but it's probably out there, I just haven't really looked yet.
NickM 17y ago
If you did the same experiment on Jupiter, then the speed of deceleration absolutely would be affected. When you hit the ground, you stop because there is a large force accelerating you upwards. On Jupiter, there is still the same upwards force on you, but a stronger downwards force from gravity, and therefore you will decelerate somewhat more slowly. Not a lot more slowly, since the upward force from the ground is much larger than the gravitational force, but it still makes a difference. On the other hand, if you're on a planet with more gravity, the ground is likely to be harder since there's more force compressing it together, but that's a whole different issue.
- graphene 17y ago
  Well, I don't know... If you postulate beforehand that the ground won't deform, that means it will match any force applied to it, including that of a plummeting experimentalist. On jupiter, the force of gravity is greater, so the force applied to the ground by the experimenter's face will also be greater, but since the ground always matches it, the decelleration will occur in the same timespan (which is in fact zero provided neither the ground nor the experimenter's body deform). If you do allow deformation, your point is valid. However, the thought experiment is still not accurate as two people of different weight performing the experiment will take different times to decellerate, due to the heavier one having gained more momentum. However, since they would have reached the ground in the same time (Gallileo's finding), they would arrive at different conclusions regarding the relative magnitudes of the Gravitational and Electromagnetic forces.

PieSquared 17y ago

And why do electrons not want to share orbitals?

graphene 17y ago
Because that's just the way they are :-) More precisely, no two electrons can ever occupy exactly the same quantum mechanical state. http://en.wikipedia.org/wiki/Pauli_exclusion http://en.wikipedia.org/wiki/Pauli_exclusion
- scotty79 17y ago
  Why the thing that keeps them from occupying the same quantum mechanical state by holding them apart (like in neutron stars) is not called force (the fifth one)? That's actually quite funny that we call 'fundamental forces' only the forces that we can't explain the source of.
deleted 17y ago
[deleted]
btilly 17y ago
Because they are fermions, not bosons. Which has to do with their spin, not their charge. See http://en.wikipedia.org/wiki/Pauli_exclusion_principle http://en.wikipedia.org/wiki/Pauli_exclusion_principle for more.

btilly 17y ago

Obviously the structure of an atom is affected by electrons not wanting to share orbitals. But isn't it electromagnetism that keeps atoms from passing through each other? And isn't it also electromagnetism that causes groups of atoms to clump in molecules, then the molecules to stick together in a crystalline pattern? (Update, the answers respectively seem to be "not entirely" and "yes".)

graphene 17y ago
If that were true, why doesn't all matter stick together in one giant gob? If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference? In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible.
- btilly 17y ago
  If that were true, why doesn't all matter stick together in one giant gob? A typical atom has a net charge of 0 with minor local variations. Therefore its attractive force drops off much faster than 1/r^2. And so electromagnetic attraction between atoms effectively disappears at any significant distance. If the structure of an atom is "obviously" affected by the exclusion principle, why would the structure of materials, made up of atoms, make any difference? It is possible for both rigid and non-rigid materials to satisfy the Pauli exclusion principle. Therefore the rigidity of things around us cannot be caused by the Pauli exclusion principle. In fact, both Coulombic repulsion and the exclusion principle are involved in the hardness of materials. I believe the Coulombic fraction is larger than 50%, but the other part is certainly non-negligible. Are you talking about volume or hardness? You are right that the exclusion principle results in volume. See http://en.wikipedia.org/wiki/Pauli_exclusion_principle#Stability_of_matter http://en.wikipedia.org/wiki/Pauli_exclusion_principle#Stabi... for verification. (That effect is much bigger than I had realized.) However it does not necessarily result in rigid things that are hard. Consider, for example, dropping a bucket of fresh water into salt water. Does the fresh water stop quickly upon impact? Obviously not. Now consider dropping a rock in the dirt. Does the rock stop quickly upon impact? Obviously it does. What is different between the two scenarios? The answer is the structure of the materials, which are held together by electromagnetic forces. Therefore it really is electromagnetic forces that quickly stop falling solid objects that hit solid things.

RiderOfGiraffes 17y ago

The Pauli exclusion principle pertains, does it not, to electrons (etc) not wanting to duplicate QM states, but is it not still the case that one atom not passing through another atom is becuase of the repulsion of the orbitals? Electrons in a single atom is a different question from electrons in different atoms. Of course, I am probably wrong, and look forward to your pointers to further reading to help me understand more.

lucifer 17y ago
Not just electrons. Take a look at Fermions: http://en.wikipedia.org/wiki/Fermion http://en.wikipedia.org/wiki/Fermion
slackenerny 17y ago
Most detailed explanation in the blogosphere is "Why Stuff is Hard" by Jim Pivarski, http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-hard/ http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-har... http://cornellmath.wordpress.com/2007/08/11/why-everything-is-hard/ http://cornellmath.wordpress.com/2007/08/11/why-everything-i... http://cornellmath.wordpress.com/2007/08/11/why-everything-is-hard/#comment-253 http://cornellmath.wordpress.com/2007/08/11/why-everything-i... Complete argument is the Dyson, Lieb and Thirring's proof of stability of matter, http://projecteuclid.org/euclid.bams/1183555452 http://projecteuclid.org/euclid.bams/1183555452 .
- anatoly 17y ago
  Many thanks for that last link.
graphene 17y ago
But if one atom "tries" to pass through another, the electrons from both atoms will be spatially very close to each other, and consequently, they will be occupying very similar states. Coulombic repulsion certainly also plays a role, but it's not the only force at work here. I don't know the relative magnitudes for single atoms bumping into each other, but for bulk material, I believe it's about 40% exclusion, 60% coulomb. Why would electrons in different atoms interact differently than electrons in the same atom? They don't, as far as I know. Furthermore, if electrons from different atoms only interacted Coulombically (repelling each other), molecules and solids would never get formed. These are difficult questions, not addressed in the clear-cut cases often presented in introductory QM textbooks. I can't think of any must-read references right now, but if you're interested in this kind of thing I know of two books on molecular quantum mechanics; one by McWeeney (methods of molecular quantum mechanics) and one by Atkins (molecular quantum mechanics), which I haven't read myself, but I hear it's good. I don't know of any free online sources for this kind of stuff, but it's probably out there, I just haven't really looked yet.

NickM 17y ago

If you did the same experiment on Jupiter, then the speed of deceleration absolutely would be affected. When you hit the ground, you stop because there is a large force accelerating you upwards. On Jupiter, there is still the same upwards force on you, but a stronger downwards force from gravity, and therefore you will decelerate somewhat more slowly. Not a lot more slowly, since the upward force from the ground is much larger than the gravitational force, but it still makes a difference. On the other hand, if you're on a planet with more gravity, the ground is likely to be harder since there's more force compressing it together, but that's a whole different issue.

graphene 17y ago
Well, I don't know... If you postulate beforehand that the ground won't deform, that means it will match any force applied to it, including that of a plummeting experimentalist. On jupiter, the force of gravity is greater, so the force applied to the ground by the experimenter's face will also be greater, but since the ground always matches it, the decelleration will occur in the same timespan (which is in fact zero provided neither the ground nor the experimenter's body deform). If you do allow deformation, your point is valid. However, the thought experiment is still not accurate as two people of different weight performing the experiment will take different times to decellerate, due to the heavier one having gained more momentum. However, since they would have reached the ground in the same time (Gallileo's finding), they would arrive at different conclusions regarding the relative magnitudes of the Gravitational and Electromagnetic forces.