# Space between objects when Normal force equals 0

1. Dec 22, 2012

### PhanthomJay

Ok, so 2 objects in contact with one another exert a normal force pair between the 2, but it is my understanding that they never 'touch' each other, rather, it is the repulsion of the outer electrons that create the electromagnetic 'normal' force.
QUESTION 1: Is there physical space betwen the 2, and how much space?

Now next scenarios, 1 object rest atops another object, and they are dropped in free fall. Or a marble is in a circular 'loop the loop' at its critical speed at the top (sq.rt (rg)). In both cases, there is no normal force, and thus, no contact between the objects, or between the marble and its track.
QUESTION 2: How much physical space now exists between the 2 objects (or between the marble and track if you like that option better)?

Thank you.

2. Dec 22, 2012

### soothsayer

I don't know that there is a really good way of figuring that out. If I were to make an order of magnitude estimate, maybe a nanometer? It depends on how much force is put on the objects, because you could physically eliminate that space and begin to break molecular bonds, etc.

3. Dec 23, 2012

### rhythm42

Taking this into consideration, if you exclude a force strong enough to do that, I believe you're left with an ever so slightly different answer depending on the objects being in relatively high external pressure (gas atmosphere maybe) or if the objects are in a vacuum.

4. Dec 23, 2012

### Staff: Mentor

It seems to me that depends on your definition of where an object "ends." Since you posted this in the Quantum Physics forum, I assume you're aware that we have to deal with probability distributions for the atomic electrons. What probability density do you consider to mark the "boundary" of an object?

5. Dec 23, 2012

### lugita15

I think for a good order-of-magnitude sense, we can take a characteristic size of the atom, like the Bohr radius or the radius at which the probability density is maximized.

6. Jan 4, 2013

### PhanthomJay

Well, I'm thinking this over and now realize that the term "empty space" lacks meaning because objects don't exist in space, but rather are spatially extended, thank you Albert. Allow me to ask a related question concerning forces between objects separated by a certain distance: we know that the electromagnetic force is huge orders of magnitude greater than the gravity 'force', both of which are forces acting at a distance, and yet, it seems that the emf force can do nothing to keep the apple from falling from the tree. Please explain .

7. Jan 4, 2013

### I like Serena

Who said so?

Charge up the apple to some significant charge, and also charge up the surface it's falling on, and you'll see it will be able to counteract gravity!

In practice positive and negative charges are always close together, giving rise to a so called dipole moment.
The related electric force falls off with the third power of the distance, leaving gravity in charge (no pun intended) on long distances.

8. Jan 6, 2013

### soothsayer

Charge is screened out over macroscopic distances, for the most part, so most everything we experience in our day to day life is neutral, and there is no EM force acting on it.

However, electromagnetic bonds are responsible for keeping things together. If and apple falls from a tree and hits the ground, the reason it doesn't just keep tunneling through the ground is because of the strength of the electromagnetic bonds between the molecules in the ground vs. the strength of gravity acting downward on the apple.

As another example, take a small magnet, and set it on a table, then try lifting up the magnet. It is very easy to do. But get a second magnet and attach it to the first, then try pulling the magnets apart...quite a bit harder! You can also easily pull a magnet off the ground using another magnet (and think about how much bigger the Earth is than your second magnet!) These will give you a good feeling for why the electromagnetic force is much, much stronger than gravity.

9. Jan 7, 2013

### PhanthomJay

OK, it does seem to me like we have 2 different types of emf interactions: one is the action at a distance non-contact emf force between electrically charged particles, and the other is the 'contact' emf force between electrons of neutrally charged particles. The former acts over great distances and the latter over tiny (Planck scale?) distances. With what little I know of the quantum world, I assume that at this scale, that both forces, along with quantum gravity, may be the same?? I mean, like Unified??

10. Jan 7, 2013

### I like Serena

From an observer's point of view there are a number of different emf interactions.
However, these are all already unified in the electric force.

- Magnetism is explained as a special form of the electric force by the theory of relativity.
- The regular electric force falls off with the square of the distance.
- A positive and a negative charge close together, generate an effective electric force (a residual force) that approximately falls off with the third power of the distance.

On smaller scale, we have the strong force that holds quarks together.
This is a force that does not fall off with distance at all.
However, since the particles that consist of quarks are color neutral, outside of such a particle we only observe a residual force.
This is the nuclear force that binds protons and neutrons together.
It falls off exponentially with distance.

In all this, gravity is not involved.