Date: Fri Aug 8 22:10:40 2008

Author: Dan Bernoulli

Subject: Re: Friction lab, depends on velocity?

Post:

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Friction is one of those nasty things that doesn't model well. We should re=
ally be teaching the Fermi model of Friction. If you can find it=2C Cliffor=
d Swartz in his book Teaching Introductory Physics: A Sourcebook=20
gives a nice short synopsis of what to expect (and some of what not t=
o expect) when covering friction. I have always enjoyed his book because it=
really gives one the "lay of the land" when it comes to teaching standard =
topics in introductory physics. He simply and succinctly tells the way it i=
s=2C not how we would like it to be. I see it is currently not available on=
the AAPT site. Hopefully it will reappear. It has been worth every cent of=
the $100 or so dollars I paid for it. It is one of the few books I paid fo=
r out of my own pocket so I could honestly call it my own.

(legal denials of connection to the author=2C publisher etc. go here : )

Dan Beeker

> Subject: [tap-l] Friction lab=2C depends on velocity?
> Date: Thu=2C 7 Aug 2008 17:29:51 -0500
> From: Cartert@cod.edu
> To: tap-l@lists.ncsu.edu
>=20
> Folks=2C
>=20
> I was getting a lab ready for the fall term and I wanted to show the
> students that the coefficient of kinetic friction doesn't depend on the
> velocity of the object. That is=2C as long as you're moving at a
> constant velocity=2C it doesn't matter what that velocity is=2C the force=
of
> friction is the same. (This ignores air drag.)
>=20
> The problem is that when I tried the lab out=2C it turned out that th=
e
> coefficient _DID_ depend on velocity=2C contrary to every intro physics
> book I've read!! It's a small effect=2C around 10%=2C but it's obvious =
and
> easily reproducible. I need some help!
>=20
> Here's what I did....I attached a vernier force probe to a short 2X4
> wooden block and then pulled it along a Pasco aluminium lab track at a
> relatively constant velocity. (I used a motion detector to check my
> velocity rate.) Okay=2C this is blank wood on aluminum so there aren't an=
y
> weird trend effects or fluids involved. But I need about 10% MORE
> force to pull at a constant 0.4 m/sec than at 0.2 m/sec. =20
>=20
> The speed very low so there shouldn't be significant air drag. I
> tried increasing the mass of the system by putting a brass mass on the
> block=2C but the effect was still there.
>=20
> So two questions:
> 1) Anyone want to explain this? Are all the physics books (and
> my lectures for the last 8 years) wrong?
>=20
> 2) Can someone propose a demo that WON'T so this effect? I'm
> hunting around and was thinking of putting felt on the bottom of the
> block=2C but I was hoping for some sage advice.
>=20
> Thanks.
>=20
> Tom=20
>=20
> ------------------------------------------
> Dr. Tom Carter
> Physics
> College of DuPage
> (o) 630-942-3346
> http://www.cod.edu/people/faculty/cartert
> =20
>=20
>=20

_________________________________________________________________
Got Game? Win Prizes in the Windows Live Hotmail Mobile Summer Games Trivia=
Contest
http://www.gowindowslive.com/summergames?ocid=3DTXT_TAGHM=

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Friction is one of those nasty things that doesn't model well. We should re=
ally be teaching the Fermi model of Friction. If you can find it=2C Cliffor=
d Swartz in his book Teaching Introductory Physics: A Sourcebook&nbs=
p=3B
gives a nice short synopsis of what to expect (and some of what not t=
o expect) when covering friction. I have always enjoyed his book because it=
really gives one the "lay of the land" when it comes to teaching standard =
topics in introductory physics. He simply and succinctly tells the way it i=
s=2C not how we would like it to be. I see it is currently not available on=
the AAPT site. Hopefully it will reappear. It has been worth every cent of=
the $100 or so dollars I paid for it. It is one of the few books I paid fo=
r out of my own pocket so I could honestly call it my own.(legal de=
nials of connection to the author=2C publisher etc. go here : )Dan =
Beeker>=3B Subject: [tap-l] Friction lab=2C depends on velocity?<=
br>>=3B Date: Thu=2C 7 Aug 2008 17:29:51 -0500>=3B From: Cartert@co=
d.edu>=3B To: tap-l@lists.ncsu.edu>=3B >=3B Folks=2C&=
gt=3B >=3B I was getting a lab ready for the fall term and I want=
ed to show the>=3B students that the coefficient of kinetic friction =
doesn't depend on the>=3B velocity of the object. That is=2C as lon=
g as you're moving at a>=3B constant velocity=2C it doesn't matter wh=
at that velocity is=2C the force of>=3B friction is the same. (This =
ignores air drag.)>=3B >=3B The problem is that when I trie=
d the lab out=2C it turned out that the>=3B coefficient _DID_ depend =
on velocity=2C contrary to every intro physics>=3B book I've read!! =
It's a small effect=2C around 10%=2C but it's obvious and>=3B easily=
reproducible. I need some help!>=3B >=3B Here's what I did=
....I attached a vernier force probe to a short 2X4>=3B wooden block =
and then pulled it along a Pasco aluminium lab track at a>=3B relativ=
ely constant velocity. (I used a motion detector to check my>=3B velo=
city rate.) Okay=2C this is blank wood on aluminum so there aren't any&=
gt=3B weird trend effects or fluids involved. But I need about 10% MORE>=3B force to pull at a constant 0.4 m/sec than at 0.2 m/sec. >=
=3B >=3B The speed very low so there shouldn't be significant air =
drag. I>=3B tried increasing the mass of the system by putting a br=
ass mass on the>=3B block=2C but the effect was still there.>=
=3B >=3B So two questions:>=3B 1) Anyone want to explain t=
his? Are all the physics books (and>=3B my lectures for the last 8 y=
ears) wrong?>=3B >=3B 2) Can someone propose a demo that WON'T=
so this effect? I'm>=3B hunting around and was thinking of putting=
felt on the bottom of the>=3B block=2C but I was hoping for some sag=
e advice.>=3B >=3B Thanks.>=3B >=3B =
Tom >=3B >=3B ---=
--------------------------------------->=3B Dr. Tom Carter>=3B =
Physics>=3B College of DuPage>=3B (o) 630-942-3346>=3B ht=
tp://www.cod.edu/people/faculty/cartert>=3B >=3B >=3B Got Game? Win Prizes in the Windows Live Hotmail Mobile Summe=
r Games Trivia Contest Find out how.
=

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