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Re: Inertia demos.

From: "Tom McCarthy" <TMccarthy@steds.org>
Date: Sat, 29 Nov 1997 14:47:29 -0500

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From: Donald E. Simanek <dsimanek@eagle.lhup.edu>
To: phys-l@atlantis.uwf.edu <phys-l@atlantis.uwf.edu>
Cc: phys-l@atlantis.uwf.edu <phys-l@atlantis.uwf.edu>
Date: Saturday, November 29, 1997 12:28 PM
Subject: Re: Inertia demos.

=20

Ok, folks, educate me. I've read all this pussyfooting around the =

subject

of inertia demos, but no one has yet said what, exactly, students LEARN
about INERTIA from such demos. I haven't a clue. What physics do they =

come

away with that's useful in other situations? Keep it simple and spell =

out clearly so my feeble brain can comprehend. Convince me that this
tablecloth demo, as usually done, has *any* educational value.

These arguments *don't* convince me, so you'll have to come up with
something better:

(1) It shows that a body at rest remains at rest until acted upon by a
force. (It doesn't show that at all, for the cloth *is* exerting a =

force,

and a considerable one, on the bottom of the objects. Yet they remain
essentially at rest. At least you hope they remain at rest.)=20

Appealing to the definition of inertia that I have been spouting for =
quite some time; inertia is the measure of an object's tendency to =
remain at rest or stay in constant motion in the presence of an =
unbalanced force, I would say that these sorts of demonstrations reveal =
the nature of mass, albeit under some very limited conditions. When I =
perform these demonstrations, I ask my students to identify the inertial =
object for which the demo was designed and the force that is acting upon =
the object. I do not go far beyond this as an analysis of the =
magnitudes of the forces, the associated impulse, etc. unless the =
particular demo calls for it. As an example, I like to perform the =
heavy balls demo where a heavy steel ball is hung from the ceiling by a =
string and a second string is hung from beneath the ball. A second ball =
is hung similarly. I use 35 pound test line to emphasize the property =
of inertia (not the principle). I pull one string very quickly and the =
other slowly. I ask the students to consider the situation before I do =
it and then afterwards they must identify the items I mentioned above. =
In this situation, impulse is a very critical element of the explanation =
but does not have to appear as such. When pulled slowly, the ball is =
placed in motion with the string, but the building remains always at =
rest. I believe that a good thinking student (not necessarily a smart =
student) would assimilate some valuable understanding off inertia from =
these parlor tricks. At the end, either through student questions or =
teacher qualification, the limitations of the se demos are presented.
=20
I perform another demo in the gym with a bowling ball (16 lb) and a =
basketball. Since the floor is very smooth and the bowling ball very =
round (no holes drilled), the bowling ball, once in motion, fairly =
nicely remains in motion. Now, if I hit the rolling ball smartly and =
perpendicularly to its motion, it will immediately change its motion and =
then continue along a new, straight path. Since the bowling ball has so =
much mass, its path is not significantly changed. With the basketball, =
however, the same rap of the hammer produces a much greater change in =
path. This, too, demonstrates inertia and is perhaps a better, though =
not as exciting a demo. What do you all think?
=20
I believe they all have some educational value, but this, as Donald =
points out depends on the investment of answering a number of critical =
questions. I brought up this topic because I started second guessing =
the worth of these fun demos after hearing someone say they aren't =
demonstrating what they were intending to. In the desire of fostering a =
better understanding, I would be willing to drop certain tricks of the =
trade and look for others to replace them. But, I need to be convinced =
they are not serving their purpose. Like the Lenz's Law demo, I have so =
much fun with this one that I'd hate to give it up if it is really much =
more complicated than I am making it out to be.

Tom McCarthy
Saint Edward's School
Vero Beach, FL=20

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<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>From: Donald E. Simanek <<A=20
href=3D"mailto:dsimanek@eagle.lhup.edu";>dsimanek@eagle.lhup.edu</A>><B=
R>To: <A=20
href=3D"mailto:phys-l@atlantis.uwf.edu";>phys-l@atlantis.uwf.edu</A> =
<<A=20
href=3D"mailto:phys-l@atlantis.uwf.edu";>phys-l@atlantis.uwf.edu</A>><B=
R>Cc: <A=20
href=3D"mailto:phys-l@atlantis.uwf.edu";>phys-l@atlantis.uwf.edu</A> =
<<A=20
href=3D"mailto:phys-l@atlantis.uwf.edu";>phys-l@atlantis.uwf.edu</A>><B=
R>Date:=20
Saturday, November 29, 1997 12:28 PM<BR>Subject: Re: Inertia=20
demos.<BR><BR> </DIV>
<DIV></FONT>><BR>>Ok, folks, educate me. I've read all this =
pussyfooting=20
around the subject<BR>>of inertia demos, but no one has yet said =
what,=20
exactly, students LEARN<BR>>about INERTIA from such demos. I haven't =
a clue.=20
What physics do they come<BR>>away with that's useful in other =
situations?=20
Keep it simple and spell it<BR>>out clearly so my feeble brain can=20
comprehend. Convince me that this<BR>>tablecloth demo, as usually =
done, has=20
*any* educational value.<BR>><BR>>These arguments *don't* convince =
me, so=20
you'll have to come up with<BR>>something better:<BR>><BR>>(1) =
It shows=20
that a body at rest remains at rest until acted upon by a<BR>>force. =
(It=20
doesn't show that at all, for the cloth *is* exerting a =
force,<BR>>and a=20
considerable one, on the bottom of the objects. Yet they=20
remain<BR>>essentially at rest. At least you hope they remain at=20
rest.) </DIV>
<DIV><BR></DIV>
<DIV><FONT face=3D"Times New Roman" size=3D3>Appealing to the definition =
of inertia=20
that I have been spouting for quite some time;  inertia is the =
measure of=20
an object's tendency to remain at rest or stay in constant motion in the =

presence of an unbalanced force, I would say that these sorts of =
demonstrations=20
reveal the nature of mass, albeit under some very limited =
conditions.  When=20
I perform these demonstrations, I ask my students to identify the =
inertial=20
object for which the demo was designed and the force that is acting upon =
the=20
object.  I do not go far beyond this as an analysis of the =
magnitudes of=20
the forces, the associated impulse, etc. unless the <FONT =
color=3D#000000=20
face=3DArial size=3D2>particular </FONT>demo calls for it.  As an =
example, I=20
like to perform the heavy balls demo where a heavy steel ball is hung =
from the=20
ceiling by a string and a second string is hung from beneath the =
ball.  A=20
second ball is hung similarly.  I use 35 pound test line to =
emphasize the=20
property of inertia (not the principle).  I pull one string very =
quickly=20
and the other slowly.  I ask the students to consider the situation =
before=20
I do it and then afterwards they must identify the items I mentioned=20
above.  In this situation, impulse is a very critical element of =
the=20
explanation but does not have to appear as such.  When pulled =
slowly, the=20
ball is placed in motion with the string, but the building remains =
always at=20
rest.  I believe that a good thinking student (not necessarily a =
smart=20
student) would assimilate some valuable understanding off inertia from =
these=20
parlor tricks.  At the end, either through student questions or =
teacher=20
qualification, the limitations of the se demos are =
presented.</FONT></DIV>
<DIV><FONT face=3D"Times New Roman" size=3D3></FONT> </DIV>
<DIV><FONT face=3D"Times New Roman" size=3D3>I perform another demo in =
the gym with=20
a bowling ball (16 lb) and a basketball.  Since the floor is very =
smooth=20
and the bowling ball very round (no holes drilled), the bowling ball, =
once in=20
motion, fairly nicely remains in motion.  Now, if I hit the rolling =
ball=20
smartly and perpendicularly to its motion, it will immediately change =
its motion=20
and then continue along a new, straight path.  Since the bowling =
ball has=20
so much mass, its path is not significantly changed.  With the =
basketball,=20
however, the same rap of the hammer produces a much greater change in=20
path.  This, too, demonstrates inertia and is perhaps a better, =
though not=20
as exciting a demo.  What do you all think?</FONT></DIV>
<DIV><FONT face=3D"Times New Roman" size=3D3></FONT> </DIV>
<DIV><FONT color=3D#000000 face=3D""><FONT size=3D3><FONT face=3D"Times =
New Roman">I=20
believe they all have some educational value, but this, as Donald points =
out=20
depends on the investment of answering a number of critical =
questions.  I=20
brought up this topic because I started second guessing the worth of =
these fun=20
demos after hearing someone say they aren't demonstrating what they were =

intending to.  In the desire of fostering a better understanding, I =
would=20
be willing to drop certain tricks of the trade and look for others to =
replace=20
them.  But, I need to be convinced they are not serving their=20
purpose.  Like the Lenz's Law demo, I have so much fun with this =
one that=20
I'd hate to give it up if it is really much more complicated than I am =
making it=20
out to be.</FONT></FONT></FONT></DIV>
<DIV><FONT color=3D#000000 face=3D""><FONT size=3D3><FONT=20
face=3D"Times New Roman"></FONT></FONT></FONT><FONT face=3D"Times New =
Roman"><FONT=20
size=3D3></FONT></FONT> </DIV>
<DIV><FONT face=3D"Times New Roman">Tom McCarthy</FONT></DIV>
<DIV><FONT face=3D"Times New Roman">Saint Edward's =
School</FONT> </DIV>
<DIV><FONT face=3D"Times New Roman">Vero Beach, =
FL</FONT>  </DIV>
<DIV>></DIV></BODY></HTML>

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