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On Aug 18, 2016, at 2:02 AM, Joseph Bellina <inquirybellina@comcast.net> wrote:
Regarding what happens when surfaces are in contact and you try to move one relative to the other, there are several possibilities. The resistance can be physical if the surfaces have some roughness and the hills and valleys impede the motion. It can be chemical when bonding between some or many of the surface atoms bond. As John pointed out for very clean surfaces that are not even atomically flat cold welding will occur. Generally both will occur since surfaces are generally not atomically flat and are covered with a surface layer that limlts bonding to small regions where the surface layer is warn away as the surfaces are pressed together
As you applied a shear force the bonds break and or the surface region deforms and the surfaces move relative to each other and rebond. as the shear force continues the bonds break and or the surface region deforms and the process repeats
When there is no relative motion we call that static friction. If the surfaces move perpendicular to each other the bonding gives us rolling friction. When the surfaces move parallel to each other that is what we call kinetic friction. In the latter two cases the motion is a microscopic studder-step, that is a series of static situations separated by microscopic motions.
I think is microscopic model is easy to understand and using it eliminates the various misstatements that have been mentioned
It also explains nicely why the normal force increases the resistance to relative motion because the surfaces are smashed together making it harder to overcome the physical barrier and by increasing the bonding
The lack if affect of area arises because in real materials only a small fraction of the surface is actually in contact so when you Increase the area you decrease the normal force in each unit of area but increase the regions for potential bonding and roughness effects. The two effects tend to cancel each other out.
Another factor will be that the energy supplies to the interact will change the local temperature and each of the factors I mentioned are highly temperature dependent. In any event the simple bonding studder-step should avoid many misunderstandings
Best
Joe
Sent from my iPhone
On Aug 17, 2016, at 3:46 PM, John Denker <jsd@av8n.com> wrote:
The big, useful idea is:
Words acquire meaning from how they are used
... not from some pithy dictionary-style definition.
===============
In support of that big idea, I offer a number of smaller,
less-constructive observations.
On 08/17/2016 05:53 AM, Bruce McKay wrote:
Our syllabus describes frcition as a force that opposes motion.
Small point: The frictional force vector is /not/ reliably
directed opposite to the total motion vector. Counterexamples
abound, as guaranteed by Galileo's principle of relativity.
On 08/17/2016 10:53 AM, Bob Sciamanda wrote:
Friction opposes the relative motion of surfaces in contact -
it opposes slipping of your feet when you walk -
it opposes slippage of the tires as a car accelerates.
When motion of the contact point is thus impeded momentum is transferable into other, allowed motion.
Small point: In those examples, i.e. static friction, the frictional
force vector is /not/ directed opposite to the relative motion vector
... because there is no relative motion.
Small point: Glue opposes relative motion, but glue is not friction.
Small point: The normal force opposes relative motion in the normal
direction, but that's not friction either.
Small point: There is such a thing as aerodynamic drag, which includes
skin friction drag as well as pressure drag. All three of those things
"could" be opposite to the motion, but they're not the same thing. Also,
there is such a thing as pressure recovery, which contributes a negative
amount of drag, so it's not opposite to the motion at all.
Small point: If you try to measure the coefficient of friction between
two very flat, very clean blocks of metal, you'll find that it's infinite.
The blocks will cold-weld, and you'll never get them apart again.
Larger point: Although it "might" be possible to come up with a definition
of friction that didn't suffer from quite so many bugs, I suggest it's not
worth the trouble. Words acquire meaning from how they are used, not from
some pithy dictionary-style definition.
In the classroom, this poses a challenge, because by the time students show
up in physics class they have been trained for more than 10 years that the
way to get a good grade is to rote-memorize the definition and regurgitate
it on the test.
My point is that the rote regurgitation approach is the opposite and the
enemy of reasoning.
Students have been told for years that reasoning is valued, but they know
this is a lie. They know this absolutely, based on years of tests. So
if you say they will /not/ be asked to give a pithy definition of friction,
they will not believe you the first ten times you say it. Still, you have
to say it anyway. Then you have to walk the walk.
Students will start out with a rough idea of what friction is, and then
refine this idea as they gain more understanding.
Let's be clear: I am not offering a "better" definition of friction. I
recommend not giving it any official definition at all. I say words are
defined by how they are used, not by some pithy dictionary-style definition.
I *some* cases, we may choose to decompose the contact force into a
projection normal to the surface and a projection in the plane of the
surface. We *might* choose to call the in-plane piece the frictional
piece. This is not based on any notion of absolute motion or even
relative motion; it's based only on the orientation of the surface.
More importantly, this isn't physics; it's just terminology, and as
such it's not guaranteed to make sense. In particular, a cog train
can climb a steep hill, not limited by friction in the usual sense.
Ditto for sandpaper in contact with sandpaper; it's not clear the
notion of static friction means anything. Ditto for the clean metal
blocks. Et cetera.
At the very least, if friction is to mean anything, it is important to
distinguish
-- static friction
-- sliding friction
-- quasi-static rolling friction
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