What are contact forces, really?

In summary, the four non-contact forces are explained through the interactions of different fields, including the electromagnetic force. Contact forces, such as pushing a door, also involve the interactions between fields and their intermediary photons. These forces are not as simple as the inverse square law and are influenced by factors such as the Pauli exclusion principle. At the atomic level, particles do not actually touch and are made up of other fundamental components.
  • #1
eightsquare
96
1
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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  • #2
It's still the interactions between fields. The electrons never touch, but come close to each other then interact through intermediary photons.
 
  • #3
Yes, contact forces are mainly electromagnetic forces - repulsion between electrons (via electromagnetic fields), if you push a door.
 
  • #4
@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?
 
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  • #5
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.
 
  • #6
@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
It is not as simple as the usual inverse square law, yes.
 
  • #8
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
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.
 

1. What are contact forces?

Contact forces are a type of force that occurs when two objects physically touch each other. These forces can either be pushing or pulling, and they are responsible for the interaction between objects in our everyday lives.

2. How do contact forces differ from non-contact forces?

Contact forces require physical contact between two objects, while non-contact forces do not. Non-contact forces, such as gravity and magnetic forces, can act at a distance without any physical contact between the objects.

3. What are some examples of contact forces?

Some examples of contact forces include friction, tension, normal force, air resistance, and applied force. Friction occurs when two surfaces rub against each other, tension is the force exerted by a string or rope, normal force is the force that supports an object on a surface, air resistance is the force that opposes the motion of an object through air, and applied force is any force that is applied to an object by a person or another object.

4. How do contact forces affect motion?

Contact forces can either cause objects to start moving, stop moving, or change direction. For example, friction can cause an object to stop moving, while applied force can cause an object to start moving in a particular direction.

5. Can contact forces be both attractive and repulsive?

Yes, contact forces can be both attractive and repulsive. For example, when two magnets come into contact, they can either attract or repel each other depending on the orientation of their poles. Similarly, when two objects are pushed together, the normal force between them can be repulsive, while the force of friction between them can be attractive.

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