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

[Anthony Lapinski <alapinski@pds.org>]

I like that Mythbusters video/explanation!

I often show this fake video (water slide jump) to my students:

www.youtube.com/watch?v=lkwh4ZaxHIA

On Wed, Aug 17, 2016 at 9:59 AM, Leon de Oliveira <leondeolive@gmail.com>
wrote:

> Mythbusters did a good piece on this and ended up showing a way to make the
> video. I would start with the faked video and have a discussion on whether
> it was real or not. Maybe have them do some trials and experiments and come
> up with reasons for their stance. Maybe discuss scaling and then show the
> Top Gear clip. Discuss any changes in their perceptions and discuss any
> changes in the experiment that could get them the results in the video. Run
> the first part of the Mythbusters clip and then check back with them. Then
> finish the Mythbusters clip to show how they did the "stunt".
>
> Mythbusters clip: https://www.youtube.com/watch?v=lK1ci50DUgc
>
> Leon
>
> On Wed, Aug 17, 2016 at 8:26 AM Bill Norwood <bnorwood111@gmail.com>
> wrote:
>
> > Bruce,
> > I'm afraid that the demo with the motorcycle is phony, because nothing
> > moved at all.
> > Bill Norwood
> > U of MD at College Park
> >
> > On Wed, Aug 17, 2016 at 8:53 AM, Bruce McKay <brumac@bigpond.net.au>
> > wrote:
> >
> > > When teaching Newton’s first law and inertia, I show the clip from Top
> > > Gear (on YouTube) on pulling a table cloth.
> > >
> > > https://www.youtube.com/watch?v=B_acz9zO0ro
> > >
> > > BMW did it with a motor bike and then Top Gear’s attempt with a Nissan
> > > GTR. Its fun.
> > >
> > > I demonstrate it by placing a 5 kg mass on a sheet of paper and slowly
> > > pulling the paper across the desk and the mass stays on the paper.
> Then,
> > > with the mass on the paper close to the edge of the desk, i pull the
> > paper
> > > very rapidly and the mass gets left behind. The explanation is then
> > really
> > > Newton’s second law as, when pulled rapidly the frictional force is too
> > > small to make the mass accelerate at the same rate as the paper.
> > >
> > > Friction is an issue beyond Newton’s first law. Our syllabus describes
> > > frcition as  a force that opposes motion. I ask how could we move if
> > there
> > > was no friction?
> > >
> > > Bruce McKay
> > > St Ignatius’ College,
> > > Riverview  NSW
> > > Australia
> > >
> > >
> > > > On 17 Aug 2016, at 12:52 PM, stefan jeglinski <jeglin@4pi.com>
> 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
> > > >
> > > > _______________________________________________
> > > > Forum for Physics Educators
> > > > Phys-l@www.phys-l.org
> > > > http://www.phys-l.org/mailman/listinfo/phys-l
> > >
> > > _______________________________________________
> > > Forum for Physics Educators
> > > Phys-l@www.phys-l.org
> > > http://www.phys-l.org/mailman/listinfo/phys-l
> > _______________________________________________
> > Forum for Physics Educators
> > Phys-l@www.phys-l.org
> > http://www.phys-l.org/mailman/listinfo/phys-l
> --
>
> The difference between being average and being extraordinary will all come
> down to you executing your expectations.
> _______________________________________________
> Forum for Physics Educators
> Phys-l@www.phys-l.org
> http://www.phys-l.org/mailman/listinfo/phys-l