Re: [Phys-l] Static vs kinetic friction

[Bill Nettles <bnettles@uu.edu>]

John,
I don't have a specific answer to your question about the role of static friction during the "pause" of a block sliding up then down. The complexity of the static friction situation can be shown quite easily with a demo. Put a block on a board, tilt the board until the block starts sliding. Then remove the block, tilt the board to different angles, placing the block on the board after tilting it. The angle at which the block starts moving after placement will usually be different (usually lower) from the first situation. Static contact time can affect static friction coefficients, which goes hand-in-hand with Joseph Bellina's comment. 

Bill Nettles
Union University

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Joseph Bellina
Sent: Monday, October 12, 2009 11:37 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Static vs kinetic friction

John,
Friction involves bonding at the microscopic level of atomically  
rough surfaces.  So several factors come into play.  The first is  
deformation of the interface due to the normal contact forces.   
Second is chemical bonding at those deformed interfaces.  So it all  
depends on how much time that takes.  And of course for finite  
contacting surfaces, those processes might go one slightly before and  
after the momentary stop.

I think you would have to make a fairly detailed chemical/mechanical  
model that might make more sense in a materials science course.

joe

Joseph J. Bellina, Jr. Ph.D.
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556

On Oct 12, 2009, at 12:17 PM, John Clement wrote:

> Friction is a very complicated subject, but most elementary physics  
> books
> have a fairly good explanation.  So in an effort to make some of the
> effective labs available even when the necessary equipment is not  
> available,
> I have been creating fairly realistic simulations.
>
> As usual I try to make the simulations general enough that they can  
> be used
> the same as actual experiments.  So I have a cart on a ramp which  
> is given
> an initial push and then is sometimes caught at the end of the  
> ramp.  I have
> included friction, but only kinetic friction.  It seems to work  
> fairly well
> and reproduces the sorts of results which one expects and uses for  
> Activity
> Based Physics, including the published ILDs.
>
> But there is the possibility of including static friction.  The  
> push is
> generally fairly strong, so static friction would only come into  
> play if the
> cart stops.  So there is one possible case where it might be  
> different with
> static friction.
>
> Does static friction come into play during the instant when a cart  
> reverses
> direction after being kicked up the ramp?  This would only have an  
> effect if
> the gravitational force component along the ramp is greater than  
> kinetic
> friction, and less than static friction.  So can the kicked cart  
> stop during
> the turnaround because of this effect?   I suspect that the static  
> friction
> may require a stationary period of time, but an instantaneous stop  
> may not
> be sufficient to fully activate the effect.  I suspect the friction  
> may
> increase slightly during the turnaround, but no to the maximum  
> static value.
> This is a tricky experiment, but perhaps someone already has  
> experience with
> the answer.
>
> I probably will not modify my simulation to take this into account,  
> but
> depending on the answer, who knows?  Incidentally simulations can  
> in some
> cases be used to produce slightly better understanding than  
> physical labs,
> so they are a viable alternative to labs.
>
> John M. Clement
> Houston, TX
>
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