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[Phys-L] Re: Quick question on static frictionHi All

Make a force diagram for the forces on the cup. This is an important=
but subtle point that is often missed in discussions of macroscopic =
forces: only those forces acting directly ON THE CUP should be consid=
ered. Forces on other objects surrounding the cup act only on those =
objects. Those forces do not act on the cup, so should not be include=
d in a diagram of forces on the cup. The force of the hand on the pap=
er doesn't count, because the hand isn't in contact with the cup. Th=
e paper IS in contact with the cup, so this contact force is included=
. =20
=20
There are three forces on the cup: its weight due to gravity, the upw=
ard normal force from the surface that it's sitting on, and the horiz=
ontal force that is causing the cup to accelerate. The horizontal for=
ce is the force of friction between the paper and the cup. Therefore,=
static friction IS making the cup accelerate. Could Serway be wrong=
? Yes....EVERY book can, and probably does, contain errors. =20
=20
What counts in determining "static" vs. "kinetic" friction is what ha=
ppens at the interface. As long as the paper and the cup move togeth=
er across the table, the horizontal force between the cup & paper is=
"static" friction. When the cup is sliding across the paper, the for=
ce between the two is then "kinetic" friction. For a cup sitting on =
a piece of paper sliding across the table, there is "static" friction=
between the cup & the top surface of the paper at the same time that=
there is "kinetic" friction between the table and the bottom surface=
of the same sheet of paper. The force between the paper and the hand=
pulling the paper is also classified as static friction.
=20
A thought experiment: What would happen if one "turned off" the forc=
e of static friction between the paper and the bottom of the cup? In=
that case, the cup should not move when the paper is pulled. One mig=
ht test this experimentally by putting some ball bearings, BB's (same=
thing), or marbles between the cup and the paper, or using a C02 or=
hovercraft puck (the "kick dis" toy, for example) in place of a cup.=
With no horizontal force, there can't be a horiz. acceleration of t=
he cup (puck). Therefore it's not the "hand" that's causing the cup t=
o move, but indeed the force of static friction between the cup and t=
he paper. =20
=20
You can't use this as an excuse to get you out of taking responsibili=
ty for a broken cup. The "hand" is exerting a static friction force =
that causes the paper to move in the horizontal direction. Therefore=
the owner of the "hand" is still responsible for sweeping up the pie=
ces, even if the force of the "hand" was not directly operating on th=
e cup.
=20
Vickie Frohne
=20

________________________________

=46rom: Forum for Physics Educators on behalf of rlamont
Sent: Thu 3/10/2005 9:26 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Quick question on static frictionHi All



I guess that's the source of my uneasiness with this. I agree
with you that my hand is the ultimate "source" of the kinetic
energy of the cup. Serway makes a general statement that static
friction does no work. But I'm finding it hard to see why the
situation where I accelerate the the cup directly with my hand,
my hand is doing work - but if I use the sheet of paper as an
intermediary to accelerate the cup, the static frictional force
is not doing work. How come my hand in contact with the cup does
work, but the paper (via static friction) does no work? Suppose I
grab the cup handle between my two fingers and only use static
friction to pull the cup? Is it me doing the work or the static
friction force?

I always find it hard to apply hard and fast "rules" to
situations with macroscopic forces - the whole concept of a force
seems to become ill defined.

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-
L@list1.ucc.nau.edu] On Behalf Of James Frysinger
Sent: Thursday, March 10, 2005 10:07 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: Quick question on static frictionHi All

Perhaps my view is simplistic but it seems to me that one can
answer
the
question by first asking what happens to the energy of the
coffee cup?
If it
is maintained at a constant velocity on a level surface, it
neither gains
nor
loses kinetic or gravitational potential energy. If there is no
rubbing of
the paper across the coffee cup, there is no gain in thermal
energy.

On the other hand, if the cup accelerates then the paper is
doing work
on the
cup, just as your hand would have if the cup had been sitting
on your
hand.

Where does the paper get the energy that ultimately accelerates
the
cup and
itself (if that occurs) and to add thermal energy to itself and
the desktop
by rubbing? From your hand.

Jim

On Thursday 10 March 2005 09:55, rlamont wrote:
Hi All,

I have a quick question related to static friction that came
up
in class today - and I just don't have time today to search
the
archives - so forgive me if I'm traveling an old path here.

If I place a coffee cup on a sheet of paper and pull on the
paper
gently enough to accelerate the coffee cup without it
slipping on
the paper, has the force of static friction done work on the
coffee cup? One of the homework problems in Serway (Physics
for
Scientists and Engineers) implies that no work is done by the
static frictional force because there is no actual
displacement
of the cup relative to the paper. I don't find that very
satisfying because the cup still gains kinetic energy and the
only force acting horizontally on it is the static friction.

Help!

Bob at PC

--
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE

http://www.cofc.edu/~frysingj
frysingerj@cofc.edu
j.frysinger@ieee.org

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