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



In terms of exposing the force-motion vs force-change in motion distinction, working with something at rest is problematic because there is no distinction in that case since the force produces both motion and a change in motion. Rather start with something moving at constant velocity and consider how to change its velocity, and then return to how it was that the original velocity was constant in the first place. The devices Dan and Rick suggest are good bets for doing this.

best,

joe

Joseph J. Bellina, Jr. Ph.D.
Retired Professor of Physics
Co-Director, Northern Indiana Science, Mathematics, and Engineering Collaborative (NISMEC)
Consultant I-STEM Network
574-276-8294
inquirybellina@comcast.net




On Aug 17, 2016, at 9:01 AM, Daniel MacIsaac <danmacisaac@gmail.com> wrote:

Exploring behaviors of high-inertia, low friction objects is traditional — bowling balls, blocks of dry ice and ride-on homemade hovercraft are all useful in my experience; googling around finds many such examples. Dan M


Dan MacIsaac, Associate Professor of Physics, SUNY-Buffalo State College
SAMC278 BSC, 1300 Elmwood Ave, Buffalo NY 14222 USA 1-716-878-3802
<macisadl@buffalostate.edu> <http://PhysicsEd.BuffaloState.edu>
Physics Graduate Coordinator & NSF Investigator for ISEP (MSP) and Noyce

On Aug 16, 2016, at 10:52 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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