Re: What is the coefficient of friction of human skin?

Edwin:
     That is interesting.  I too, have placed coins, and various objects 
on my arm and tilted it to acute angle until it fell, but never really 
considered what the frictional coefficient between my skin and the object 
was.  Hmmmmm.
     You have a couple of questions here, and I will try and get to all of 
them, but first, some of what you are witnessing is not really because of 
the frictional coefficient in the classical sense.  When you place a coin 
on your arm for example, there are lots of factors that determine how it 
will 'stick.'  There is a certain amount of tackiness, due to the moisture 
in/on your skin that actually will help the coin stick due to properties 
of surface tension in water, and not directly due to the frictional 
coefficient of your arm/ penny.  This said, We'll continue.
     Friction between two surfaces is due to the bonding and interaction 
of molecules of the objects at those places (atomically) where the 
surfaces are in very close contact.  It is obviously in the opposite 
direction of the applied force, in your coin on the arm example, gravity.  
An important thing to remember is that the force of friction is 
independent of the area of contact, and is proportional to the force 
exerted by one surface onto the other.  Static friction, is different that 
kinetic fricion, and they differ in definition by whether the objects are 
moving  or stationary.  Kinetic friction is less than static 
friction.
     I could not find information on the friction coefficient for human 
skin v. XXX.  I suspect this is due to the fact that there is such a 
variety of tpyes and areas of epidermis that getting an accepted number 
would be difficult.  As you already surmised, lots of things will affect 
the friction between the skin and an object.  The most important is 
probably humidity.  As your skin becomes more moist, its friction 
coefficient increases substantially.  I do not mean, wet, because then you 
are gliding on water, not skin, but when your skin is clammy moist, as 
opposed to dry.  Hair too, affects, the coefficient, and for the most 
part, it tends to decrease it, as hair probably exerts less friction than 
typical skin.  I am not certain that race would have a large impact, but 
weather certainly would affect it, again, mostly due to mositure content.  
I handilly left my Newton force meter at home, or I would have done a few 
experiments on my own (Presumably average)arm.  I hope this helps, but you 
can certainly examine some physics texts and look for tables listing 
friction coefficients for various objects and try and approximate what 
skin would be like.  Thanks for your question, it was funny.  


-Matt-