# What are contact forces, really?

by eightsquare
Tags: contact, forces
 P: 96 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?
 P: 17 It's still the interactions between fields. The electrons never touch, but come close to each other then interact through intermediary photons.
 Mentor P: 10,499 Yes, contact forces are mainly electromagnetic forces - repulsion between electrons (via electromagnetic fields), if you push a door.
P: 96

## What are contact forces, really?

@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?
Mentor
P: 10,499
 Quote by 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?
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.
 P: 96 @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?
 Mentor P: 10,499 It is not as simple as the usual inverse square law, yes.
 Sci Advisor PF Gold P: 11,082 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.
 P: 96 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.

 Related Discussions Introductory Physics Homework 1 Introductory Physics Homework 1 Introductory Physics Homework 1 Introductory Physics Homework 3 General Physics 1