When two objects meet, what happens? What is the question?

[dogjones]

I have been pondering this for ages - bugging my friends, acquaintances or people I vaguely know who are into physics (such as high-school physics teachers.) I haven't actually asked this here because in a sense I haven't figured out what the question is (apologies to Douglas Adams.)

It starts with a minor idiosyncrasy of mine. If I recline on a sofa, I have a habit of throwing my lighter (yes, I smoke; that's my major habit; I acknowledge I'm a pariah) up in the air above my head. But I don't just throw the lighter randomly, oh no. I try to make it hit the ceiling. But not just hit the ceiling, oh no. I try to make it meet the ceiling with as little force as possible. i.e., the ideal throw would be the lighter touching the ceiling, but with no force whatsoever. So the lighter would not rebound off the ceiling; it would touch it, and then return to earth as it would if it had never touched it - but it would touch it nonetheless.

Now, I am no physicist. I'm not particularly good at maths - apart from solving the odd quadratic equation, and I haven't done that since I was 15. So I know I am thinking in a kind of linguistic way which is probably the wrong way of thinking of things. So bear with me.

What I want(ed) to know is: Is it possible for two objects to 'meet' - and yet transfer no energy (dunno if this is the right word - is 'force' better?) to each other? I know I am thinking in a very Newtonian way here; in particular I am thinking of his third law - for every action there is an equal and opposite reaction. So can two objects meet and not 'react'?

I asked lots of people and most of them said no - if two objects 'meet', or 'hit each other', then there has to be force transferred between the objects at the point of contact. But I don't think this is the case. Assume there is no ceiling; I throw the lighter up in the air. The force I exert on the lighter is more powerful than gravity, temporarily. So the lighter travels upward. Eventually, gravity takes over (the energy I have 'given' to the lighter runs out?) and the lighter travels back downward. There is surely a point in between, where the lighter's velocity is 0, yes? So can that point, where the velocity is 0, coincide with it 'touching' another object, ie the ceiling?

I would say yes. I tried to conceptualise the problem by imagining that the only objects in the universe are two golf balls and two rubber bands. The rubber bands are unbreakable, and the maximum distance they can stretch before recoiling is the radius of the universe. One golf ball is attached to one of the rubber bands, which in turn is anchored to the 'west' side of the universe. The other ball is attached to the other rubber band, which in turn is anchored to the 'east' side of the universe, i.e., completely the complete opposite side. Assume that whatever the rubber bands are anchored to has no gravitation, or anything - the rubber bands basically represent the only force acting on the golf balls apart from what is to come, which is: the golf balls are then propelled toward each other at the same time, by the Hands of Yaweh or whatever. So the golf balls both meet perfectly in the middle of the universe, before the rubber bands' forces take over and start pulling them back where they came from. The golf balls touch each other, but at the point they touch each other, each ball's velocity is 0. Mathematically possible? I still wasn't sure.

Then I thought of the whole thing backwards. Start from the two balls touching, and then hypothetically apply forces (like the rubber bands) to pull them apart. They begin by not exerting force, or reacting against each other... so if it is possible to start from that point, then it is possible to get to that point. So case closed - two objects CAN 'meet', at a velocity of 0, and not 'react' against each other.

But the thing is (and this is where the Douglas Adams-esque 'what is the question' comes in) - I realised I actually don't know what I really mean when I say two objects 'meet', or 'touch each other'. What happens at the microscopic/molecular/atomic/smaller-even-than-that level when two objects 'meet'? There must be forces going on at that level that I have no clue of. And different types of objects probably have different ways of 'meeting' at the subatomic level. The world is a complex place. Which is why I don't really know what the bleedin eck I'm asking. Simply - what happens when two macro-level objects 'touch' each other? Can they touch each other and not affect each other at all? Or am I asking the wrong thing? Am I thinking in entirely the wrong way? Should I just stop smoking?

These are the questions that have been keeping me up at night - sad eh? If anyone could shed any light, either in the form of an answer or how I should think of the question better, or even just to improve my vocabulary, it would really be very cool.

42.

Cheers

 

[toddjh]

But the thing is (and this is where the Douglas Adams-esque 'what is the question' comes in) - I realised I actually don't know what I really mean when I say two objects 'meet', or 'touch each other'. What happens at the microscopic/molecular/atomic/smaller-even-than-that level when two objects 'meet'?

When we say that two objects "meet" or "touch," what we're usually referring to is the resistance created when the electrons in one object get too close to the electrons in the other object and they repel each other.

So, with that in mind, I think your friends are right: if two objects "touch," then energy is being transferred from one object to the other via the electromagnetic force. If no energy is being transferred, then they're not really touching. You can arrange it so that the amount of energy transferred is arbitrarily small, but it can't be zero.

However, there are two things that make it more complicated than that. The first is that, even when we push our hands down on a desk, we're not really touching it. The atoms in our hand are getting very close to the atoms in the desk, but the electromagnetic force is incredibly strong and it stops any of the electrons from actually touching each other. Even when we're pressing down, our hands are sort of floating above the surface of the desk on a thin cushion of electomagnetic repulsion. Kinda neat, when you think about it.

And the second thing is that there's always a certain amount of this repulsion between objects. Even when your hand is six inches above the desk, there's a tiny amount of repulsion -- so tiny that you couldn't possibly detect it, but it's still there. So determining when two objects "meet" isn't a simple yes/no proposition; it's a matter of choosing an arbitrary dividing line for the point where we decide the repulsion is significant. Intuitively this is very easy for us to know what "significant" means -- we can feel it -- but mathematically it's harder.

 

[Soapy Sam]

toddjh- The same holds true for atoms within the lighter and the ceiling- they are not touching , but are attracted by chemical bonding forces, which are in equilibrium with the repulsive forces, so the lighter stays together as a recognisable entity.

If we can increase the attractive forces at the surface of the lighter, the dynamic of the equilibrium alters and it may become one with the ceiling.
Sticking chewing gum on it would do.

 

[Unnamed]

I had a chance to play with an atomic force microscope. It's an expensive toy with very sharp tip (hundreds of atoms) that slides across the surface and "feels" the interaction between the tip and the surface.

Unless they atract each other, the transition is smooth: as the tip gets closer, it gets repelled more and more, so there is no clear point where they "touch". If the surface attracts the tip, it comes closer until it "touches", that is, until attraction and repulsion equilibrate each other.

Several different kinds of surface forces exist. Electrostatic interaction is long-ranged due to the inverse-square law, while van der Waals force^WP is very short-ranged, to give two examples. So different pairs of surfaces may "touch" at different distances (for a given definition of "touch").

In any case, "touching" is intrinsically an atomic-scale phenomenon, so you should not think of your objects as hard and sharp.

To answer your question, I can't think of a meaningful definition of "touch" or "meet" that does not involve a small but nonzero amount of interaction.

 

[Unnamed]

What happens at the microscopic/molecular/atomic/smaller-even-than-that level when two objects 'meet'? There must be forces going on at that level that I have no clue of. And different types of objects probably have different ways of 'meeting' at the subatomic level. The world is a complex place.

Besides what people already said, this may be useful (but it barely scratches the surface): Intermolecular force^WP

Reality is usually much stranger than we imagine. To quote Nobelist Wolfgang Pauli, "God created the bulk, the devil created the surface".

 

[69dodge]

I asked lots of people and most of them said no - if two objects 'meet', or 'hit each other', then there has to be force transferred between the objects at the point of contact.

Suppose the two objects are not moving or anything; they're just sitting next to each other on the table. Then what do those people say? Can two objects sit next to other, just barely touching, without exerting forces on each other?

Strictly speaking, as others have said in this thread, the answer is no. But, for all practical purposes, the answer is yes. (Ok, ok, most practical purposes...) And the important point is, the answer is the same whether you place two lighters next to each on the table or whether you throw one up to the ceiling at exactly the right speed.

I'd say your intuition is spot on.

Of course you should stop smoking, though.

 

[JamesM]

toddjh- The same holds true for atoms within the lighter and the ceiling- they are not touching

I don't get this. I would be interested in what definition of 'touch' and 'atom' you're using here.

 

[Horatius]

Strictly speaking, as others have said in this thread, the answer is no. But, for all practical purposes, the answer is yes. (Ok, ok, most practical purposes...) And the important point is, the answer is the same whether you place two lighters next to each on the table or whether you throw one up to the ceiling at exactly the right speed.

My impression is he's not looking for "practical purposes", but for some Platonic ideal of "touching" and "zero force". As others have said, when you consider the atomic (and sub-atomic) nature of matter, the answer is no, there will always be some force, although said force may be so small as to be impossible to differentiate it from all other forces at work.

 

[Iamme]

Now that you are pondering this...ponder or try THIS:

If you lay on your back, bored out of you gourd, and want to try something.... Try Rick Geshay-ing ( I went to school with him, seriously) a super ball off the ceiling, straight above you and see if you are quick enough to catch it before it hits you in the head.

 

[fuelair]

However, there is a transfer of force: there are Newtonian laws at work that in essense say when the lighter pushes against the ceiling, the ceiling pushes back against the ceiling with the same amount of force and/or m1v1 (mass of lighter X its' velocity) pre-collision + m2v2 (mass of ceiling X its' velocity [0 m/s]) precollision) =(postcollision) (m1v1)' + (m2v2)' (v2 extremely near 0 but not 0 m/s).