# The cause of Friction between objects

1. Aug 17, 2007

### ron_jay

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)=$$\mu$$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.

2. Aug 17, 2007

### mgb_phys

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. Aug 17, 2007

### jobyts

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. Aug 17, 2007

### mgb_phys

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. Aug 18, 2007

### ron_jay

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 - $$\mu$$
Roughness coefficient - $$\alpha$$
contamination coefficient - $$\beta$$
and a certain constant - K

therefore, Friction(F)= K$$\mu$$ $$\alpha$$ $$\beta$$

Could this be?(I doubt)

6. Aug 18, 2007

### rewebster

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. Aug 18, 2007

### andyp

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. Aug 18, 2007

### mgb_phys

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. Aug 18, 2007

### rewebster

what is 'melting' and 'refreezing' actually though?

10. Aug 18, 2007

### mgb_phys

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. Aug 18, 2007

### rewebster

but the iron was cut -not broken

12. Aug 18, 2007

### rcgldr

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. Aug 18, 2007

### mgb_phys

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.

Last edited: Aug 18, 2007
14. Aug 18, 2007

### rcgldr

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?