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

[Bruce McKay <brumac@bigpond.net.au>]

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
>
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