Re: [Phys-L] inertia and the tablecloth demo

[stefan jeglinski <jeglin@4pi.com>]

I think this is largely my objection to the tablecloth experiment. While 
both this example and the tablecloth can be discussed in the context of 
inertia, I don't think there's much there for the student – what are 
your actual experiences with students? If I was a student and was told 
this was a "demonstration of inertia and what it means," I'd feel 
underwhelmed. All of the questions about fast and slow and what causes 
the string to actually break are still there and unanswered. As a 
student I'd say "the bowling ball has the same inertia in either case - 
how is that the determining factor?" To the skeptical student, I'd argue 
that you'd quickly leave appeals to inertia behind and talk about 
acceleration and stick/slip friction (in the case of the table cloth) 
and acceleration and tensile strength in this example.

Again, I'm not saying that inertia has no place in this discussion – it 
certainly does, esp as a starting point. but I feel like a better 
/demonstration/ of inertia alone is closer to "here, try to move this 
bowling ball and this ping pong ball and tell me what you think."

[see next response]

Stefan Jeglinski


On 8/17/16 12:20 PM, Richard Tarara wrote:
> BTW:  A really simple demo that deals with inertia and especially 
> emphasizes the 'change in motion' aspect of the physics is to simply 
> tie a string to a 1 kg mass.  Pull upwards on the string and the mass 
> rises.  Jerk hard on the string and the string breaks.  Discuss....  
> [Be sure to test that the string will break, especially if you wrap 
> the string around your hand before the jerk.  ;-]
>
> rwt
>
>
>
>
> On 8/16/2016 10:52 PM, stefan jeglinski wrote:
> > For the first time this Fall, I'm teaching a class in "How Things
> > Work" to a group of non-science majors. This, I have never done
> > before, and it's a bit daunting to know that I need to connect to them
> > in a different way from STEM majors.
> >
> > At any rate, we're using the book How Things Work, by Bloomfield, and
> > he explains that the dishes remain on the table when you whisk a
> > tablecloth from under them "because of inertia." He expands only
> > slightly, and I do get what he's saying, but I feel like this isn't
> > the best way to try to get Newton's First Law across. This
> > "experiment" depends sensitively on factors such as the acceleration
> > of the tablecloth, and the static and kinetic coefficients of
> > friction. If you use a looong tablecloth you will probably get in
> > trouble. To the contrary, it seems that friction is one of the reasons
> > that people don't really get the First Law: "objects in motion stay in
> > motion" but virtually everything that you slide across a table doesn't
> > do this. Isn't this regarded as one of the reasons that the
> > force-and-motion connection became so ingrained?
> >
> > Like I said, I do understand how one could use this demo to discuss
> > the first law, but it seems to me that a glider on an air track or a
> > puck on an air table are more instructive given a finite class time.
> > What does everyone think?
> >
> >
> >
> > Stefan Jeglinski
> >
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