What are contact forces, really?

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Discussion Overview

The discussion revolves around the nature of contact forces, particularly in the context of electromagnetic interactions and quantum mechanics. Participants explore how forces operate at the atomic level when objects are pushed against each other, questioning the concept of "contact" in physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that contact forces are primarily electromagnetic forces, with repulsion between electrons playing a key role when pushing objects like doors.
  • Others argue that electrons never actually touch, instead interacting through fields and intermediary photons, raising questions about the nature of contact forces.
  • One participant questions why there is a net repulsion if atoms are neutral, suggesting that a deeper understanding involving quantum mechanics and the Pauli exclusion principle is necessary.
  • Another participant notes that the interactions between electrons are complex and not simply described by classical laws, indicating additional factors at play when electrons are very close to each other.
  • A viewpoint is presented that challenges the existence of contact forces, emphasizing that particles do not truly touch and that there is significant space between atomic nuclei.

Areas of Agreement / Disagreement

Participants express differing views on the nature of contact forces and the interactions at play, with no consensus reached on the fundamental mechanisms involved.

Contextual Notes

The discussion highlights limitations in understanding the quantum mechanical aspects of atomic interactions and the definitions of contact forces, with some participants indicating confusion over the mathematical descriptions provided.

Who May Find This Useful

This discussion may be of interest to students and individuals exploring the concepts of forces in physics, particularly those curious about atomic interactions and the implications of quantum mechanics.

eightsquare
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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?
 
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It's still the interactions between fields. The electrons never touch, but come close to each other then interact through intermediary photons.
 
Yes, contact forces are mainly electromagnetic forces - repulsion between electrons (via electromagnetic fields), if you push a door.
 
@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:
eightsquare said:
@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.
 
@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?
 
It is not as simple as the usual inverse square law, yes.
 
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.
 
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.
 

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