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Why do objects stay together?

  1. Jun 19, 2013 #1
    I thought this when I was pondering how, when you sit on a chair, you're butt isn't really touching the chair; it's just interacting at short distances with its electrons. After thinking about that, I realized that your torso isn't touching your butt either, it's just interacting at short distances with its electrons as well. So, if your torso and your butt are pretty much as connected to each other as your butt and the chair, then why is it that when you stand up from sitting in the chair, your butt comes with you, but the chair doesn't? I understand that metallic bonds hold together metals sometimes, but those can't possible hold together your whole body, right, or can metallic bonds operate on large chunks of nonmetals and what-not mixed together as well? Also, if something like metallic bonds do hold together your body, then why wouldn't they attach you to the chair once you sat on it?
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  3. Jun 19, 2013 #2


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  4. Jun 19, 2013 #3


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    If my torso was touching my butt, I would be shaped like a pretzel. What is this fixation you have with things touching your butt?
  5. Jun 20, 2013 #4
    I was kind of hoping that someone wouldn't just give me a link to a huge page on Wikipedia and say, "Here, the answer to your question is probably somewhere inside." I'm not lazy, but I'm not going to waste my time reading over a whole Wikipedia page that may not even contain the answer to my question. If I wanted a Wikipedia page, I would have brought one up myself. Is there a specific part of that page that you think will answer my question? If you find a web page that you think is interesting and that relates to my question, then I would gladly read it as long as you include a reference to the exact part of the page that relates to my question. Do you have something useful to share, like any knowledge about the different types of bonds and/or how they might relate to sitting in a chair? If you do, then it would be great if you could share that knowledge with me, because I am very curious about this chair thing.

    Okay, fine, maybe I should have said that your waist touches your butt, but that isn't the point of my asking this question (and besides, your butt technically doesn't touch anything either; it's just interacting at short distances with other things' electrons). I am genuinely puzzled. It would be helpful if you could reply to my post with some information that could help me better understand what is happening when you sit down and sit up. After all, that is what this thread is all about.

    Also, neither of these posts have even partly answered my question. I would very much appreciate it if someone who knew the answer to my question would post it.
  6. Jun 20, 2013 #5


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    Ah, but that IS touching! I suppose the answer to your original question is that your butt doesn't stick to the chair because the atoms and molecules interact but do not form chemical bonds. Without bonding two atoms do not stick together.
  7. Jun 20, 2013 #6
    Technically speaking, no. Touching is when two objects are so close together that there isn't any space in between them. I guess it does actually depend on the way you define "touching," in which case, I have no right to say your wrong, but you have no right to say I'm wrong either.

    So every single atom in your body is chemically bonded? What about when the chair turns out to be a butt-eating chair and it eats off my butt. Does that break the chemical bonds between my [STRIKE]torso[/STRIKE] waist and my butt? I thought that cutting/biting off was a physical change, but wouldn't it have to be a chemical change if it involves breaking the chemical bonds between my butt and my waist?
  8. Jun 20, 2013 #7


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    Boundary layers, that is what separates different bodies. Generally (on metals) a layer of oxide forms which protect the bulk of metal from further reaction. Undoubtedly similar layer exist on non metals.
  9. Jun 20, 2013 #8


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    Technically speaking, yes, it is. There is no other form of 'touching'. Things do not get so close that there is no space between them. Even defining something like the size and boundaries of an atom is problematic since it relies on a probability map of where the electrons might be at any given time.

    Practically all of them. I'm sure there are a few floating around that aren't bonded. Keep in mind that your body is a complex object made up of trillions upon trillions up cells, all surrounded by a network of connective tissue and fluids, in which trillions of other things like bacteria and viruses stroll through like they own the place. In such an environment I doubt there are any quick, simple answers.

    The simple answer is yes. However you'd have to look into everything that actually holds your body together and see how it all works to get a full understanding. Your body isn't a solid object with only one type of bond between everything. It has many different ways of staying intact and all operate differently. My knowledge on the subject is very limited, so I can't give a 100% definitive answer.

    I'm not sure what the distinction is between 'physical change' and 'chemical change' is in this circumstance.
  10. Jun 20, 2013 #9

    Simon Bridge

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    The questions seem to have started with how things stay together to how they stay apart.
    It's quite a big subject and a lot of the answers are more about exploring what is meant when we say that two bits are part of the same object ... what do we mean when we say that two things are touching - and so on. A lot of confusion comes from being imprecise about these things - but being precise will make for too much writing. So what we do is provide a bunch of rules for which there will be some exceptions.

    Very generally, seperateness and togetherness are emergent properties of underlying electromagnetic interactions.
    There is a sense in which we can say that nothing is really separate from anything else ... being careful about the definition of "really". But there is also a more immediate sense in which I like to think the table I'm sitting at is not part of me and the table owner encourages this way of thinking. A lot of this thread, therefore, is in the context of this way of thinking.

    We can do a lot by thinking of the common understandings and investigating what happens. i.e. a smooth surface that we see is the strong scattering region for the ambient light averaged over many detection events in our eyes and processed in the visual cortex before becoming part of our conscious awareness ... the link between physics, biology, chemistry, and consciousness being an, as yet, unsolved problem.

    When we say two objects are touching, then actually measure the gap very accurately, we will discover that it is not zero ... it's just small enough for the description "touching" to make sense... usually that means the gap is too small to see with the eye, or by some, otherwise useful, measuring process.

    The old saw is about why one object does not pass through another ... both are made of atoms, and are mostly empty space (it goes) and there is plenty of room for the atoms to pass each other, so why can't you push your hand through the table or walk through walls?

    Practically all the things we think of, in every day terms, as surface properties are about electrons ... objects like your hand and the table stay separate in normal circumstances because the electrons of your hand repel the electrons in the table ... the repulsion increases as they get closer and, classically, it gets infinite for zero separation: so there is no way you can exert enough force with just your muscles to push your hand through the table.

    Something like a biological organism is basically a complicated bag of stuff, with bits and bobs all tangled up.
    Every atom in your body is chemically bonded to some other atom in your body ... but not all the molecules are chemically bonded to each other, there are cells floating in fluid in your veins and arteries for example. When you get beyond chemistry it's more about which bag the stuff is in or how the fibers are wrapped around each other.

    Bags contain stuff for the same reason you cannot push your hand through the table.

    Some stuff is stickier than others - look up the origin of friction and adhesion: it's a whole field of study by itself - but it boils down to how the electrons are arranged close to the classical surface of the stuff. The stickiness is how fibers can get wrapped around each other without having zero separation between any of their parts.

    When the killer chair eats your butt, it does not have to break chemical bonds - it just has to tear the fibers apart.
    In general, this may involve breaking weak bonds and adhesion and so on just because bodies are very complicated.

    And so on and on - it is a very big subject.
    For more in-depth you should be reading about the theory of complex systems (chaos math for example - and cellular automata).
    In these forums we can handle simpler systems like two blocks of metal being rubbed together - makes for less typing ;)
  11. Jun 21, 2013 #10


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    Thanks Simon, that is exactly what I wanted to say but couldn't explain.
  12. Jun 21, 2013 #11
    Hmm. So what about the chair? Is the wood of my chair bonded? (Really it's not a wooden chair, but it isn't a man-eating chair either). Are so many everyday objects bonded so that when they are pushed down by gravity, they don't act like a liquid and 'melt' to the floor? Why do solids retain their shape? Does every solid have a chemical bond running through itself? So, does this mean that there is a special 'solid bond' that bonds all solids together? Or is it just friction somehow acting on atoms? How would friction act on atoms? I just can't imagine a molecule or an atom acting as something other than an individual particle unless it has some special type of chemical bond to 'stick' it to something else.
  13. Jun 21, 2013 #12
    There different types of solids, in terms of forces that keep them together.
    Some are similar to chemical bonds (like the covalent crystal of diamond) other are more specific to solids (like the "metallic" bond in metals).
    There are also ionic crystals where the ions are held together by electrostatic forces and van der Waals crystal with very week electric dipole forces.

    I don't think that the distinction between "chemical" bonds and other types of bonds is really relevant for your question.
  14. Jun 21, 2013 #13


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    Of course. Similar to our body, there are many different types of mechanisms that give wood its strength and allow it to hold its shape. Yet all rely on different types of bonds between atoms at the very basic level.

    Yes. However I think it's important to understand that atoms/molecules in a liquid typically have weak bonds between themselves too. A perfect example is water. A water molecule consists of one oxygen and two hydrogen atoms bound in such a way that makes the entire molecule polar, meaning that one side is positively charged and the other side is negatively charged. This allows water molecules to have weak bonds between their positive and negative sides and is the reason water is a liquid as such a high temperature compared to many other types of compounds. If it did not have these bonds water would melt and boil at much lower temperatures than it currently does.

    Some solids, such as metals and crystals, have a single type of bond between their atoms, but most have a mix of different types of bonds. This is especially true of complex objects such as wood, the human body, and your chair as a whole.
    See this article for more: http://en.wikipedia.org/wiki/Bonding_in_solids

    No, it's not friction. Friction is different. It is the force that resists relative motion between two objects. It would be better to make a new thread if you want to understand what friction is.

    You are correct. Without a bond, an atom does not act like anything else but a single atom. For example, a helium atom is very very stable and will not bond with anything else under normal circumstances. Because of this, helium does not form molecules and never partakes in chemical reactions. It is always a monatomic gas, meaning it is always found as single atoms in a gaseous state except under the most extreme of circumstances.
  15. Jun 21, 2013 #14
    Okay, I understand now. I thought that there would be some reason why objects stayed together that had nothing to do with chemical bonding. I hadn't realized that chemical bonding was so widespread. Just imagine, the atoms in every human are bonded (well, maybe not every atom, but most of them), especially those of Double 'O' Seven.:tongue:
  16. Jun 21, 2013 #15


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  17. Jun 21, 2013 #16
    I appreciate what has been said in this thread, but would like to add that a glue manufacturer told me that we don't really need glue to bond two flat surfaces together if the mating surfaces are perfectly flat (But in reality, we don't achieve that, usually)
  18. Jun 21, 2013 #17

    Simon Bridge

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    Did you read my last response?
    See if you can attempt an answer to your own question in those terms ... that way I can tell what you are learning and how you are thinking, and I can adjust the form of the answers to suit.

    Right now it just looks like you are throwing out random questions without regard to the answers you have received so far. It can be fun just throwing questions out and getting some answers back - but the idea of these forums is to learn to find your own answers.

    i.e. you have already, repeatedly, been told about the different ways that solid objects can become solid. You appear to be stuck on the idea of being "bonded" somehow, but don't seem to notice the different ways of being "bound". Did you attempt to look up the things I suggested? What did you find out?
  19. Jun 21, 2013 #18
    How does that work?
  20. Jun 21, 2013 #19

    Simon Bridge

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    @pongo & swegner99: two surfaces have to be very well matched at the atomic level to bond just from being pressed together - and wood is too varied to do that. But it is true - press two uniform "surfaces" together and they can weld together ... in, say, two lumps of metal A and B, A and B...

    Electrons like to be close to atomic nuclei but separate from each other ... if it is hard to get an electron to move away from some place, it is said to be bonded to that place. How hard it is, is the strength of the bond.

    The electrons in metal A are attracted to the atomic nuclei in A, the electrons closest to the atomic nuclei are bound only to particular nuclei, electrons farther out bind to several nuclei - effectively binding the nuclei together (look up: covalent bond) and, in a metal, these electrons can actually get shared between all the nuclei in the lump.

    The shared electrons are repelled from each other - but still attracted to the bulk material, so they sit in a cloud around the "outside" of the lump.

    When another lump of metal, B, gets closer, it's electrons will encounter the electrons of A before they get close enough to experience the nuclear charges deeper down. So the electron clouds repel each other... so the two lumps of metal stay apart. But you could apply a lot of force, making the two lumps come close enough together than the electrons of B can feel the attraction from the atoms of A and vice versa. If that happens you can get electrons shared between the two lumps and they stick together.

    The two lumps don't have to be metals - and the exact process depends on what the material is.

    The pressing-together description, above, IRL will produce a lot of heat and materials tend to melt together rather than just stick - but look up "vacuum bonding".

    Something like wood is a mixture of different kinds of material all tangled up in each other so it gets quite difficult to talk about. Pressing two bits of wood together ... I'd expect either that they'd burst into flame or some resin will melt and flow making a glue. With something like your hand and the table (my example prev) I'd expect the table and the hand to break before you could diffuse the atoms of your hand through the table.

    But for OPs purposes it is probably best to stick to ordinary everyday processes for ordinary everyday words.

    (@swegner: see if you can also follow the advise I gave to OP ... the information had already been supplied in the thread, you just needed to apply it. If you don't try, we cannot tell where we went wrong in our answers and it makes it harder to explain it to you.)
    Last edited: Jun 21, 2013
  21. Jul 27, 2013 #20
    Sorry, but I have been busy needing to do other things. I should have known that I wouldn't have much time to look into this stuff. I was just confused because I thought that cutting something in half should be a physical change, which means no chemical bonds are broken, but from what I understand from the posts on this topic, that doesn't seem to be the case. Things stick together because, in a way, they're bonded, right? It seems to me that "chemical v. physical" isn't part of the problem. So, solids usually bond through electron sharing, right? Does this mean when something turns solid, its atoms usually start to share electrons so that they become in a definite shape, or is that only in metals? I have read your replies, but I haven't looked into it much past that. I hope that that's okay.
  22. Jul 27, 2013 #21

    Simon Bridge

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    It doesn't have to be the case - but it can be that you just separate two halves that just happen to be very close together.

    My emphasis

    Define: "usually". Ionic bonds are pretty common.

    Objects can stick together through adhesion too.

    Like I said before:
    ... the exact process depends on what the material is.
    "material properties" is a very big subject.

    Metallic yeah - take a look at the kinds of bonds in solid salt or water ... also look into stuff like wax and plastics.

    It's a good idea to somehow indicate how you have taken the replies.
    Remember, nobody can see your reaction.
    Read a few of the other threads and see how others do it.
  23. Jul 28, 2013 #22
    I just don't see how substances would bond in a way that doesn't involve electrons (at the atomic level). There are ionic bonds, which involve electrons switching atoms; metallic bonds, which involve electrons whizzing throughout a substance without a particular atom claiming them; and then there are covalent bonds, which involve two atoms sharing electrons (I know I said electron "sharing," but I really mean bonding involving electrons). I am sure there are other ways for atoms to bond, and there are plenty of subatomic bonds, but the world seems to be dominated by bonds involving electrons.

    Yes, but don't the two surfaces have to be solid due to some sort of electron bonding first in order for them to bond together? Also, some types of adhesion involve electron bonding:

    Also, since objects are solid because of bonds, doesn't a substance have to be bonded to at least two other substances in order to ensure that all substances are bonded together? I just don't get how every solid in the world is, well, solid. Since everything can turn into a solid with enough pressure and with low enough temperatures, how is every substance able to achieve "solid-ness"?
  24. Jul 28, 2013 #23

    Simon Bridge

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    They can't - they just don't have to share the electrons.

    Yes - that is not being disputed.
    The article you cite shows you different forms of adhesion.

    No. Not all bonds have the same mechanism. The devil is in the details.

    You also seem to be mixing up different models ... by "substance" do you mean atoms or compounds or do you mean "wood" vs "metal"?

    Why would anyone expect otherwise?
    Just using the junior high-school particle model of matter - you can make anything a solid by stopping the particles moving around. You do that by cooling it down or by holding them tightly. That the particles in question are, at core, atoms with a distribution of charges that can be separated off just adds details to that picture... for instance, why there is a state-change at all.

    Of course some things require quite extreme conditions to become "solid" - like helium. The result is that the "solid" has some odd properties which make calling the state "solid" maybe a little misleading. Look it up.

    At what education lever are you trying to understand this?
  25. Jul 29, 2013 #24
    Sorry for being ambiguous. I'm not trying to get into complicated things like computers and living things and wood. In this thread, I usually mean substances as in elements or compounds, and I usually refer to a 'substance' as any element or compound.

    If you cool a material into a solid in a rectangular prism shape, and then you place it spanning a length like a bridge, then the particles won't just fall like a fluid into the river. I want to know why that is. The atom of the substance at the bottom has some force acting upon it to balance out the force of gravity. I thought that I was being told that that force came from the bonds that it had with other forces. If this isn't the case with all substances, then what is holding those atoms up? I'm just wondering what force could hold those atoms in place for every known substance in the Universe, because, like you said, you can turn anything into a solid by cooling it down or pressurizing it (or a solid-like state at least).

    Are you referring to a BEC? Does this mean that helium has no true 'solid' state?

    I have not taken high school chemistry. I have still been able to understand what you are saying, if that is what you want to know.
  26. Jul 29, 2013 #25

    Simon Bridge

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    OK - atom-atom interactions tend to be different from molecule-molecule interactions and molecule-atom interactions.

    Because you have cooled it into a solid of course!
    OK OK see below:

    That's right - when an atoms stay close to other atoms we say the atoms are "bonded" to each other. Some effort is required to separate them.
    There are different ways that atoms may become bonded to each other.

    You will notice, though, that a structure like a bridge may not be made out of a single slab of a substance - two solids may be joined by drilling holes through them and fitting a third solid through the hole ... those two solids are now bound by the third one.

    What I am trying to get you to realize is the variety of possible bonds.

    In previous replies you have been jumping around material types and bond types and getting confused: the confusion comes from the changes in types. A description for one will not apply to another. i.e. you talked about sharing electrons for eg ... but that's not the only way to bind two atoms.

    Define "true solid". What would a "false solid" be like?
    You didn't look up "solid helium" did you?

    No - and you do not appear to be understanding me.
    I don't care how far you got in school - I need to know where you want me to pitch the replies.
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