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



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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