The cause of Friction between objects

In summary, friction is a puzzling yet useful force that is still not fully understood, but we do have equations for it. It is caused by the interaction of minute ridges and disturbances on the surface of materials, as well as electromagnetic forces. These forces can cause materials to bond together, as seen with two smooth surfaces without any contamination. However, surface contamination and roughness can greatly affect the degree of attraction and ultimately the friction between materials. The equation for friction may include factors such as bond strength, roughness coefficient, contamination coefficient, and a constant. However, there is still doubt about whether this equation is complete. Additionally, phase changes and the formation of new surfaces are also factors in friction. Overall, friction is a complex force
  • #1
ron_jay
81
0
One of the most puzzling things yet useful is friction.Although we haven't totally understood it very well, we have certain equations for them

F(limiting)=[tex]\mu[/tex]N
[as F(limiting) is directly proportional to the weight of the body)

But, what is the main cause of this force, Is it because of the interaction of the minute ridges and disturbances or is it electromagnetic in nature.If it is electromagnetic in nature we yet don't have a concrete equation for it.
 
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  • #2
It's electromagnetic.
When you put two pieces of material together ,the atoms on the surface on one are just as strongly attracted to the surface of the other piece as they are to their neighbours.
In fact if you put two very smooth surfaces with no oil and dirt together they can vacuum weld into one piece.

You can probably model the degree of atraction given the surfacr roughness and bond strength of the material but in reality it would be strongly effected by any surface contamination.
 
  • #3
mgb_phys said:
It's electromagnetic.
When you put two pieces of material together ,the atoms on the surface on one are just as strongly attracted to the surface of the other piece as they are to their neighbours.
In fact if you put two very smooth surfaces with no oil and dirt together they can vacuum weld into one piece.

You can probably model the degree of atraction given the surfacr roughness and bond strength of the material but in reality it would be strongly effected by any surface contamination.

You mean, if I cut an iron rod into two, and if I put the two pieces together, with nothing absolutely in between, they will become one piece again?
 
  • #4
Yes if the edges were smooth and clean enough!
In practice the iron would have deformed plastically so the ends don't match up and you would have layers of oxidation, atmospheric pollution and dust on them almost instantly.

It is a very real problem in making machines that work in vacuum where it is hard to keep oil/grease on surfaces and in high precision optics where you put very flat and extremely clean surfaces together.
 
  • #5
that's fair enough to say that they would come together if those conditions of no dust etc. are met with. If we know that it is electromagnetic in nature, we should then be able to furnish a complete equation which includes all the various factors.According to what you said that the factors are dependent on bond strength of the material, the surface roughness and contamination then:

Bond strength - [tex]\mu[/tex]
Roughness coefficient - [tex]\alpha[/tex]
contamination coefficient - [tex]\beta[/tex]
and a certain constant - K

therefore, Friction(F)= K[tex]\mu[/tex] [tex]\alpha[/tex] [tex]\beta[/tex]

Could this be?(I doubt)
 
  • #6
mgb_phys said:
It's electromagnetic.
When you put two pieces of material together ,the atoms on the surface on one are just as strongly attracted to the surface of the other piece as they are to their neighbours.
In fact if you put two very smooth surfaces with no oil and dirt together they can vacuum weld into one piece.

You can probably model the degree of atraction given the surfacr roughness and bond strength of the material but in reality it would be strongly effected by any surface contamination.

I remember a teacher 'cutting' a ice block that was suspended on its ends in two with a weighted piano wire and the weighted wire dropped to the floor after going through the ice block, but the ice remained a 'block' and still suspended on its ends.

(not quite the same as an iron rod, but...)
 
  • #7
yes this is all good saying that in theory this could happen in the perfect conditions ... but we all know that every atom can Never come in contact with an other. the negitive electrons on the out side of every atom will repell the elctrons of an other so they will never touch.
 
  • #8
rewebster said:
I remember a teacher 'cutting' a ice block that was suspended on its ends in two with a weighted piano wire and the weighted wire dropped to the floor after going through the ice block, but the ice remained a 'block' and still suspended on its ends.
That's not the same thing, the metling point of ice decreases under pressure so when the wire presses down a thin small line of ice melts under the wire and the refreezes above it after the wire passes through the water.
This is how skates work, they can move easily because they are actually running on a thin layer of water between the ice and the blade.
It is possible to be too cold to ski - when the pressure from the blade isn't enough to lower the melting point below he ambient temperature.
 
  • #9
mgb_phys said:
That's not the same thing, the metling point of ice decreases under pressure so when the wire presses down a thin small line of ice melts under the wire and the refreezes above it after the wire passes through the water.
This is how skates work, they can move easily because they are actually running on a thin layer of water between the ice and the blade.
It is possible to be too cold to ski - when the pressure from the blade isn't enough to lower the melting point below he ambient temperature.

what is 'melting' and 'refreezing' actually though?
 
  • #10
A phase change (the technical term for melting/freezing) and forming new surfaces are similair in some ways.
In melting kinetic energy is needed to overcome the already formed bonds and allow the material to become disordered. In breaking something you have to put in energy to create surfaces.
Beyond that the energy behaviour and the equations are different enough that it isn't a very useful analogy - good thinking though!
 
  • #11
but the iron was cut -not broken
 
  • #12
Minute ridges or electromagnetic forces
Both are factors. Most surfaces aren't that smooth at the molecular level, and there are pits and valleys in the surfaces of most materials, and this affects the coefficient of friction. As mentioned on this link, the surface condition is very important.

http://www.school-for-champions.com/science/frictioncoeff.htm

As far as bonding from simple contact, I'm not sure if there are any solids that do this. If you place a steel block on a steel plate, there will be friction, but the two pieces will not bond to become a single piece.
 
  • #13
As far as bonding from simple contact, I'm not sure if there are any solids that do this. If you place a steel block on a steel plate, there will be friction, but the two pieces will not bond to become a single piece.
Try telling that to Nasa engineers trying to persuade antennae to unfold after they vac welded together, or someone who just out two very expensive lamba/50 optical flats together.

It is deliberately used to make extremely accurate items such as hollow cubes used to measure density standards - it's a good way of joining pieces without inducing any deformation or stresses.
 
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  • #14
Jeff Reid said:
As far as bonding from simple contact, I'm not sure if there are any solids that do this. If you place a steel block on a steel plate, there will be friction, but the two pieces will not bond to become a single piece.

mgb_phys said:
Try telling that to Nasa engineers trying to persuade antennae to unfold after they vac welded together, or someone who just out two very expenive lamba/20 optical flats together. It is deliberately used to make extremely accurate items such as hollow cubes used to measure density standards - it's a good way of joining pieces without inducing any deformation or stresses.

So there are some subtances, as posted, I wasn't sure if any solids do this. Glass is a special case, though since glass isn't considered to be a normal solid (it deforms slowly over time, so it's in a partial liquid state). Still the case of the vac welded antennas seems to be an example.

So why don't most solids bond from contact?
 

FAQ: The cause of Friction between objects

What is friction and why does it occur?

Friction is the force that resists the relative motion between two objects in contact. It occurs because of the microscopic irregularities present on the surface of objects, which get interlocked with each other when they come in contact.

What factors affect the amount of friction between objects?

The amount of friction between objects is affected by the type of materials, the weight of the objects, and the roughness of the surfaces in contact. The force applied and the surface area in contact also play a role in determining the amount of friction.

Can friction be reduced or eliminated?

Friction can be reduced by using lubricants, such as oil or grease, between the surfaces of objects. It can also be minimized by using smoother or more polished surfaces. However, it cannot be completely eliminated due to the existence of microscopic irregularities on the surface of objects.

How does the direction of force affect friction?

The direction of force does not affect friction. Friction always acts in the opposite direction of the applied force, regardless of its direction. This is known as the law of friction.

What are the practical applications of friction?

Friction has many practical applications in our daily lives. It allows us to walk, drive, and grip objects. It is also essential for braking in vehicles and for the functioning of machines. It can also be harnessed for useful purposes, such as starting fires and creating traction for sports activities.

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