1For extremely smooth surfaces cold welding takes place which increases the friction between the bodies.....e.g mirror...hence they remain in contact and don't slip over each other.
We know that friction between two surfaces is due to the uneven nature of the surface of the
bodies.
But in case of 2 plane mirrors will there be any friction ????? they have perfectly smooth
surfaces. The how can we keep one mirror over another ( their plane surfaces remaining in contact ,
Still some force is needed to separate them) Why??????
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14 Answers
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24Whatsoever smooth surface u create their will be always roughness at microscopic level.....
Their is no frictionless surface on earth
19hehe....aveek and swastika......cold welding reminds me of raja sir.....this was his pet concept in the chapter on "friction" !
62I am not very sure if cold welding is the right word to be used...
cold welding is a technique used to weld two surfaces and reminds me of my course "casting forming and welding"
1bollam na reason-ta thik ache.......
http://en.wikipedia.org/wiki/Cold_welding
Cold or contact welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum.
Aro beshi jaante hole organic-ke jigyesh kor...
1The process is analogical with atom-atom contact....meaning cold-weld together and try to form a single piece of object....etakei cold-welding process-e kaje lagano hoi..
62there is difference between joining of surfaces and between friction...
when they are joined they cant be seperated without some kind of fracture..
in cold welding generally there is a fusion between the two surfaces... the top and the lower one.. but when glass is kept on the top of another, there is no fusion....
btw...
THe more correct reason comes from molecular activity... if you think of it in terms of molecules.... and the interaction between them at a molecular level
62Summary
* Definition
o Friction is the force between surfaces in contact that resists their relative tangential motion.
o "Relative tangential motion" is a fancy way to say "slipping".
o Its direction is opposite the relative velocity (or intended velocity).
* Types
o Dry Friction
+ The resistive force between clean dry solid surfaces.
+ The phenomena one normally associates with the word friction. Friction is normally synonymous with dry friction.
o Viscous Friction
+ The resistive force between surfaces in relative motion through a fluid (liquids & gases).
o Rolling Resistance
+ The resistive force experienced by rolling objects.
+ Since rolling does not does not necessarily involve slipping, rolling resistance is not really a form of friction.
* Factors affecting dry friction
o Dry friction is directly proportional to the normal force between the two surfaces in contact.
o Dry friction depends on the materials in contact. This factor is measured by the quantity known as the coefficient of friction which is …
+ the ratio of the friction force to the normal force.
+ unitless
+ always greater than 0
+ usually less than 1 for most everyday materials
o Dry friction is subdivided into two types.
+ Static friction …
# occurs when the two surfaces in contact are not in relative motion; that is, when one surface is stationary relative to the other surface,
# varies in strength from zero (when no external force is trying to force slippage) to some maximum value (just before slippage occurs)
+ Kinetic friction …
# occurs when two surfaces in contact are in relative motion; that is when one surface is slipping or sliding across another surface,
# is always weaker than the maximum static friction.
* Factors that don't affect dry friction
o Friction is largely independent of surface roughness (despite what you may have read in other textbooks).
+ Protrusions or rough spots may provide microscopic ledges where one surface can rest upon another and apply a normal force. This is not friction.
+ The friction associated with sandpaper is no greater than the friction associated with quartz. Friction and abrasion are different phenomena.
+ Ice, glass, and rubber can all be made smooth but ice has a low coefficient of friction, glass a medium coefficient, and rubber a high coefficient. The material is what determines the amount of friction, not is surface texture.
+ Sanding a slippery surface may increase its friction by removing the low friction surface material and exposing an underlying high friction material.
o Friction is independent of speed once an object is moving.
+ Faster does not mean more friction.
Copied it from an online source....
I hope this explains some of what you want...
The main reason is moleuclar forces of attraction are higher in case of a glass...
1Yes sir....atom-to-atm contact increases in extremely smooth surface ....it's hard to break the molecular bonds (not a correct word to be used) and therefore the friction formed is greater........this concept is used in cold welding - right ?
62but there is a huge difference between the two things...
I for one would definitely refrain from using the word.. (even though a lot of things are similar as well)
1i refrain frm using the word cold welding too.......
let us consider the case that there is friction between two surfaces........there is an iron table on which an iron box is kept....there will be some force required to move it.....now if we replace the iron box by a magnet or the table by a magnetic one, then there will be iron-magnet attraction force acting between the two surfaces and surely the fotrce reqd. for slipping will be much higher than previous case.....which cannot be denied......
now let us consider the case when surfaces are absolutely smooth....in this case the molecules of the two surfaces become so smooth that there exists inter molecular attraction between the two surfaces........this is similar to the iron-magnet attraction....and so the force required to separate absolutely smooth surfaces is not zero....
am i correct??????????
1there is no question of friction here.if two perfectly dry glass pieces are taken then they will be easily separable.
if they are wet then due to surface tension and adhesive force a large force =2t/d is needed to separate them.as d tends to 0 force tends to infinity.here d is the separation between the glass pieces.