# What are contact forces, really?

1. Jul 9, 2013

### eightsquare

The four non-contact forces are explained with the help of different fields and their interactions. What about contact forces? Why does the door shut when I push it? Is it a simple transfer of momentum? If it is, do the electrons collide?

2. Jul 9, 2013

### Harry Wilson

It's still the interactions between fields. The electrons never touch, but come close to each other then interact through intermediary photons.

3. Jul 9, 2013

### Staff: Mentor

Yes, contact forces are mainly electromagnetic forces - repulsion between electrons (via electromagnetic fields), if you push a door.

4. Jul 9, 2013

### eightsquare

@mfb: Why is there a net repulsion to move the door? An atom being neutral, shouldn't there be no net force till the electrons touch? And what happens then?

@Harry Wilson: How exactly do they interact?

Last edited: Jul 9, 2013
5. Jul 9, 2013

### Staff: Mentor

The positive charges are deep inside the atom - if two atoms come close to each other, the electrons will interacting significantly first. Electrons are not billiard balls, they cannot "touch" each other.
A more detailed description needs quantum mechanics, the Pauli exclusion principle and the resulting Lenndard-Jones potential.

6. Jul 9, 2013

### eightsquare

@mfb: I don't really understand those links(I'm in 10th grade). I went through them but I didn't get all the equations. For now, can I conclude that there is an additional factor other than electromagnetic repulsion and attraction when electrons are very close to each other?

7. Jul 9, 2013

### Staff: Mentor

It is not as simple as the usual inverse square law, yes.

8. Jul 9, 2013

### sophiecentaur

There's no such thing as a contact force because nothing actually touches anything else. Take two bound atoms and there is a vast space between their nuclei. Try to push them together and the force is away, try to separate them and the force is together. There is a Potential well at their equilibrium position. (Separation of nuclei)

If you tried to make a door out of Helium Atoms they would not bond together at all so your door would deform 'very easily' lol. Even a door made of water would flow until you froze it.

Even the nucleons are not billiard balls - they have an 'effective diameter' but that doesn't mean you can actually assign them a definite edge. They are made up of other fundamental components. Geometry at that level is a different concept, I think.

9. Jul 10, 2013

### eightsquare

Okay sweet. So particles never really 'touch'. I'll do some reading on the Pauli exclusion principle. New day, new mindset, let's hope I get it this time.