Date: Thu, 09 Oct 2003 17:08:41 -0600

Author: Jerry DiMarco

Subject: Re: friction is a drag!

Post:

I think we need to be careful about what we say when a particular
equation is inadequate for describing a real world situation. This
discussion on friction is a good example. The problem, as stated by
others, is that mu is not constant under all conditions, therefore
calculations using it may be misleading. Yet statements made earlier
appear to fault the equation.
The effect of these statements if they are made in the classroom is
to cast doubt not only on the equation, but also on the entire
textbook. Given that it is already difficult to get students to crack a
book, it doesn't help to unnecessarily cast doubt on the text. In this
case, wouldn't it be more accurate to say that mu is not constant for all
conditions, and when conditions change material properties, you may have to
use different values for mu?

Regarding walking on ice, I read the article cited by Matt. That
research was news to me but it makes sense that the top layers of ice might
be more weakly bound. But when the hockey players were talking about slow
ice, they were referring to rough ice and the buildup of ice chips and
slush. This contrasts with the scientist's explanation of slow ice being
the result of "sloshing through more of these water-like layers". How
thick are these layers? If he's referring to molecular layers then there's
a big difference between the two descriptions of slow ice.
I think the greater impact of the thickness of the water-like layers
is how it affects the surface area of the blade in contact with the
ice. It makes sense that harder ice (fewer water-like layers) would not
allow the blade to sink in as much, causing the blade to glide more on the
edges alone (less contact with the hollow-ground center). Borrowing a
principle from pivots on points and knife-edge balances, I'll bet a blade
on its edge has less friction than a blade in full contact with the
ice. At first, this notion seems to violate the non-dependence of friction
on surface area, but I think there are other factors to consider.

Now back to the question of walking on ice, which I think is a
different problem. In my experience walking on cold (dry) ice is easier
than walking on ice that is beginning to melt (wet). Also, smooth, level
ice is easier to walk on than rough ice. I agree that stability and contact
area are important factors. The stability issue is pretty obvious, but the
new research results force one to rethink the contact area issue. Prior to
this I would have said less contact area means more surface melting. Now I
would ask, if the interface between ice and air causes the formation of
water-like layers, what does the interface between ice and a shoe sole
cause? More layers - therefore more slippery?
Interesting that you mentioned skating on glass. The Serway text has
a table of coefficients of friction. Ice is one of the lowest of course,
glass is not even close. Also, the coefficients for ice (static vs
kinetic) have the greatest difference...

Jerry

At 05:00 PM 10/7/2003, you wrote:
>Paul's right. Ff = mu Fn is a rough approximation at best, and materials
>for which mu is constant over a large range of normal force values are the
>exception rather than the rule.
>
>I would add that if one of the materials is significantly plastic (rubber
>tires, for example) then it doesn't work at all.
>
>On Tuesday, October 7, 2003, at 03:15 PM, Paul Nord wrote:
>
>>In reality, friction DOES depend on surface area. With ice and racing
>>tires there is a big heating effect with a small contact area. This
>>changes the material properties a lot. The linear friction term
>>introduced in physics texts is only true in a limited number of
>>situations where the speed and weight are not too large.
>>
>>Paul
>>
>>On Tuesday, October 7, 2003, at 04:43 PM, Anthony Lapinski wrote:
>>
>>>Since friction does not depend on contact area, I ask my students why race
>>>car tires are so wide. (stability, heat absorption) I also ask why tires
>>>have treads (to channel water), while race tires are bald "skids."
>>>
>>>What about walking on ice? Walking on a rough patch certainly seems more
>>>slippery than a smooth patch. Does this have to do with stability and
>>>contact area, or something else?
>>>
>>>It has been stated in many texts that ice skating works because of
>>>pressure melting. I recently read that a pressure of about 140 atm is
>>>needed for bulk surface melting of ice, and that this is much more than
>>>you get from sharp skates. So what's really going on here? You certainly
>>>can't skate on a smooth piece of glass!
>-----------------------------------------------------------------
>Dr. Eric Ayars
>Assistant Professor of Physics
>California State University, Chico
>ayars@mailaps.org

<><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

Jerry DiMarco
Manager of Lecture Demonstrations and Instructional Labs
Montana State Univ., Physics Dept.
Bozeman, MT

Our Motto: "There's a demo in there somewhere."
From Daryl@DarylScience.com Thu Oct 9 16:55:12 2003
Reply-To:
From: "Daryl L. Taylor"
To:
Subject: RE: mercury incident
Date: Thu, 9 Oct 2003 19:38:58 -0400
Message-ID:
MIME-Version: 1.0
Content-Type: text/plain;
charset="us-ascii"
Content-Transfer-Encoding: 7bit
X-Priority: 3 (Normal)
X-MSMail-Priority: Normal
X-Mailer: Microsoft Outlook IMO, Build 9.0.6604 (9.0.2911.0)
In-Reply-To:
X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2727.1300
Importance: Normal
X-MailScanner-Information: Please contact the Helpdesk @ 6266 for more information
X-MailScanner-ASU-mta2: Found to be clean
X-MailScanner-SpamCheck: not spam, SpamAssassin (score=0, required 7)
Sender: owner-tap-l@listproc.appstate.edu
X-Listprocessor-Version: 8.2.10/991025/16:55 -- ListProc(tm) by CREN

It's an artificial scare for sure. Sorta like 'global warming is our fault'.
OSHA via the EPA has directed by simple decree that Hg is a deadly thing and
must be treated as if it will produce the anti-christ and suck us all into
the void if spilled on the floor. Facts are facts. Policy and popular notion
are simply another animal. The 'masses' read about these topics in the mass
media and logs it into their collective memory. They can't believe that
Saddam had bad weapons, but this they'll believe.

The teacher in David's account below, with 20+ years of exposure, would've
had the negative effects of long term exposure if any. (Actually, most older
science teachers DO exhibit effects of some kind of poisoning. Tired.
Ornery. Lethargic at faculty meetings. Don't remember kid's names. Don't
remember wife's names.) The kids are safe. Period. I'm one of those 'old
timers' who poured Hg into my hands AND MOUTH, to show neat little
properties. (Back then we 'didn't know any better'... What hump?) So, what
is the long-term effect? Does the term 'lead' ring a bell? You ain't falling
over dead by eating a lead pendulum bob. Unless, of course, you eat one
every day for 25 years. (But, then again, if you do that, you have bigger
problems than lead poisoning.)

We, in the trenches, should know better. If you want a challenge to logic,
look in the OSHA/EPA safety tables. Isopropyl alcohol is listed as a bad
thing. See
>. To paraphrase, "Do not allow contact with skin". Their words? "Effects on
Humans: Isopropyl alcohol is an irritant of the eyes and mucous membranes.
By analogy with effects seen in animals, it may cause central nervous system
depression at very high concentrations [Hathaway et al. 1991]. Exposure to
400 ppm isopropyl alcohol for 3 to 5 minutes resulted in mild irritation of
the eyes, nose, and throat; at 800 ppm, these symptoms were intensified
[Hathaway et al. 1991]. An oral dose of 25 ml in 100 ml of water produced
hypotension, facial flushing, bradycardia, and dizziness [Hathaway et al.
1991]." I want to know who was the Einstein who swallowed this stuff to give
them this info.
What is it's sorta common name? Rubbing alcohol. It is actually illegal in
most ('most' that I know of anyway...) schools even to have it since it has
appeared on the EPA 'list'. Sorta makes alot of my density demos dangerous,
eh? I guess I should stop filling my mouth with it then spitting it out thru
a lit match, huh?

I just had a semi-long chat with 2 retired chemists who just sort of
shrugged their shoulders at this type of story. Hg is it's fluoride forms
ARE bad and quickly absorbed. Pure Hg liquid is just plain innocent of the
charges.

And Dr. Frysinger's stories brought back fond memories of scraping the paint
off 'nite clocks' and smearing it on my eyelids to scare my little brother
in the middle of the night. It worked! Of course, my eyelids are gone,
but...

Daryl L. Taylor, Fizzix Guy
PAEMST '96
Internet Educator of the Year '03
Williamstown HS, NJ
Engineering Academy, Rowan University NJ
www.DarylScience.com
609.330.9571

-----Original Message-----
From: owner-tap-l@listproc.appstate.edu
[mailto:owner-tap-l@listproc.appstate.edu]On Behalf Of David Kardelis
Sent: Thursday, October 09, 2003 5:22 PM
To: tap-l@listproc.appstate.edu
Subject: RE: mercury incident

I'm not that old but, all my professor told stories of moving a cabinet
or removing the floor boards when a building was torn being torn down
and filing piles of Hg. The Hg being there for 20+ years, exposing them
and students to Hg vapors, which now for some reason will make you mad
in minutes while the years of exposure had no noticeable effect on the
previous generations. They also talk about playing with Hg pouring it
through their hands etc. While not the best idea, they all seem to be
fine and living to ripe old ages despite the massive exposure to Hg.
Hell half the demos and apparatus they used gave exposure to Hg. Why
the recent hype about Hg dangers. Hg in pure form is relatively safe,
it used to be used as a laxative for a long time in the 1800's. Hg
salts on the other hand can be quite poisonous-- I believe the hatters
of old used HG salts and the long exposure drove them mad thus the idea
of the Mad Hatter.

dave

David Kardelis, Ph.D.
david.kardelis@ceu.edu
Chairman, Dept of Chemistry and Physics 435-613-5258
College of Eastern Utah
435-613-4201 (fax)
451 E 400 N
Price UT 84501

-----Original Message-----
From: Steve Wonnell [mailto:wonnell@pha.jhu.edu]
Sent: Thursday, October 09, 2003 10:20 AM
To: tap-l@listproc.appstate.edu
Subject: mercury incident

A high school student runs off with 250 ml of mercury.
For details, see:

http://www.washingtonpost.com/wp-dyn/articles/A474-2003Oct8.html

From rlaw@uidaho.edu Thu Oct 9 17:12:43 2003
Date: Thu, 09 Oct 2003 16:56:41 -0700
From: Russell Lawrence
Subject: RE: mercury incident
In-reply-to:
To: tap-l@listproc.appstate.edu
Message-id:
MIME-version: 1.0
X-MIMEOLE: Produced By Microsoft MimeOLE V5.50.4522.1200
X-Mailer: Microsoft Outlook IMO, Build 9.0.2416 (9.0.2910.0)
Content-type: text/plain; charset=us-ascii
Content-transfer-encoding: 7BIT
Importance: Normal
X-Priority: 3 (Normal)
X-MSMail-priority: Normal
X-MailScanner-Information: Please contact the Helpdesk @ 6266 for more information
X-MailScanner-ASU-mta3: Found to be clean
X-MailScanner-SpamCheck: not spam, SpamAssassin (score=0, required 7)
Reply-To: tap-l@listproc.appstate.edu
Sender: owner-tap-l@listproc.appstate.edu
X-Listprocessor-Version: 8.2.10/991025/16:55 -- ListProc(tm) by CREN

Gentlemen,

I am refreshed to hear that I am not the only one that thinks the danger of
mercury is overemphasized by the officials. Frankly, it is disappointing to
me that it is unaquirable and forbidden to use in demonstrations.

The only case of severe effects that I have heard about resulting from the
inhalation of mercury vapor was from the old-timers in the gold mining
industry. Mercury is attracted to gold and silver. Therefore it is and has
been used for years to collect and separate it from the other heavy metals.
Most of the mercury used to be recovered by squeezing it through a chamois
cloth, but the gold and silver still had a coating of mercury. The
resulting mix is called amalgam and in the old days, to recover the mercury
coating and to clean the gold and silver, the retort process was simply to
heat the amalgam in the center of a tent and let the mercury vapor come off.
Since the tent walls were cold, the mercury would condense on the inside of
the tent walls and drip back into cups placed in the appropriate location.
Prior to knowing that mercury vapor was dangerous, the person performing the
retort would sit inside the tent. After doing this enough times, brain
damage occurred. One story I can't forget is of a miner who shot himself
after running around naked one day.

Of course, this was more than a century ago, and the direct exposure to
mercury vapor was frequent and copious.

Retorts nowadays are sealed and safe.

I am only 35 yet my father had a jar of mercury (for amalgamation of
gold/silver) that I dipped my finger in when I was a child a few times. My
mother made me wash my hands afterwards so I hope I will not start running
around the department naked!

Anyway, I think that this emphasis about the dangers of mercury exposure (in
liquid form) and around conscientious and informed people, is
disproportionate and unjustified. An extension of the philosophy will
preclude all potentially dangerous demonstrations.

My 22 cents.

Russell Lawrence
Lab/Demo Specialist
rlaw@uidaho.edu
(208)885-2730

-----Original Message-----
From: owner-tap-l@listproc.appstate.edu
[mailto:owner-tap-l@listproc.appstate.edu]On Behalf Of David Kardelis
Sent: Thursday, October 09, 2003 3:22 PM
To: tap-l@listproc.appstate.edu
Subject: RE: mercury incident

I'm not that old but, all my professor told stories of moving a cabinet
or removing the floor boards when a building was torn being torn down
and filing piles of Hg. The Hg being there for 20+ years, exposing them
and students to Hg vapors, which now for some reason will make you mad
in minutes while the years of exposure had no noticeable effect on the
previous generations. They also talk about playing with Hg pouring it
through their hands etc. While not the best idea, they all seem to be
fine and living to ripe old ages despite the massive exposure to Hg.
Hell half the demos and apparatus they used gave exposure to Hg. Why
the recent hype about Hg dangers. Hg in pure form is relatively safe,
it used to be used as a laxative for a long time in the 1800's. Hg
salts on the other hand can be quite poisonous-- I believe the hatters
of old used HG salts and the long exposure drove them mad thus the idea
of the Mad Hatter.

dave

David Kardelis, Ph.D.
david.kardelis@ceu.edu
Chairman, Dept of Chemistry and Physics 435-613-5258
College of Eastern Utah
435-613-4201 (fax)
451 E 400 N
Price UT 84501

-----Original Message-----
From: Steve Wonnell [mailto:wonnell@pha.jhu.edu]
Sent: Thursday, October 09, 2003 10:20 AM
To: tap-l@listproc.appstate.edu
Subject: mercury incident

A high school student runs off with 250 ml of mercury.
For details, see:

http://www.washingtonpost.com/wp-dyn/articles/A474-2003Oct8.html
From mkossover@mail.hockaday.org Thu Oct 9 17:45:42 2003
Message-id:
Date: Thu, 09 Oct 2003 19:29:05 -0500
Subject: Re: mercury incident
To: tap-l@listproc.appstate.edu
From: "Marc Kossover"
References:
In-Reply-To:
MIME-Version: 1.0
Content-type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-MailScanner-Information: Please contact the Helpdesk @ 6266 for more information
X-MailScanner-ASU-mta3: Found to be clean
X-MailScanner-SpamCheck: not spam, SpamAssassin (score=0, required 7)
Reply-To: tap-l@listproc.appstate.edu
Sender: owner-tap-l@listproc.appstate.edu
X-Listprocessor-Version: 8.2.10/991025/16:55 -- ListProc(tm) by CREN

Check out the October 1972 (v. 142, no. 4, pp. 507-527) article in
_National Geographic_ on mercury, titled "Quicksilver and Slow Death."

Look especially at the picture on the bottom of page 508, where a mine
worker is floating in a pool of mercury. Only a tiny part of him is
submerged. I suppose it is too slippery to walk on.

If anyone wants a scan of the page, I'll find a site to post it briefly.

"Today thousands of products -- from thermometers and light switches to
pesticides -- depend on its unusual properties. But recent tragedies,
caused by rising levels in the environment, have shown that mercury can be
a deadly servant."

Marc "Zeke" Kossover
The Hockaday School

From mkossover@mail.hockaday.org Thu Oct 9 17:55:18 2003
Message-id:
Date: Thu, 09 Oct 2003 19:39:09 -0500
Subject: Re: mercury incident
To: tap-l@listproc.appstate.edu
From: "Marc Kossover"
References:
In-Reply-To:
MIME-Version: 1.0
Content-type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
X-MailScanner-Information: Please contact the Helpdesk @ 6266 for more information
X-MailScanner-ASU-mta3: Found to be clean
X-MailScanner-SpamCheck: not spam, SpamAssassin (score=0, required 7)
Reply-To: tap-l@listproc.appstate.edu
Sender: owner-tap-l@listproc.appstate.edu
X-Listprocessor-Version: 8.2.10/991025/16:55 -- ListProc(tm) by CREN

Russell Lawrence writes:
>Gentlemen,

With all due deference and understanding, let me point out that women do
subscribe to Tap-L as well.

Marc "Zeke" Kossover
The Hockaday School

From gofigure@ucsc.edu Thu Oct 9 18:16:37 2003

Back