I do this demo with two large, clear plastic pitchers of water placed side-by-side. On these pitchers I have placed a horizontal sheet of metal (an old "Customer Parking Only" sign that I bought for $5 at a hardware store). To make stands for my eggs I took two empty film-canisters and butted the closed ends together and taped them (with clear packing tape) and I place one stand over the center of each pitcher. It setting all of this up I take care to be sure that the edge of the metal sheet goes beyond the edge of the table. Then I take a broom and place the broom straw against the floor with my foot on the straw, the broom handle pulled back away from the edge of the metal sign- if I let go of the broom handle it moves quickly and smacks the metal sheet, sending it flying. The eggs, of course, end up in the water. (If this is too muddled and unclear, email dward@uu.edu and I'll share with you a video of my doing this with the items I describe.)
This demo elicits applause about 98% of the time.
Peace.
David Ward
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Message: 1
Date: Tue, 16 Aug 2016 22:52:53 -0400
From: stefan jeglinski <jeglin@4pi.com>
To: Phys-L@Phys-L.org
Subject: [Phys-L] inertia and the tablecloth demo
Message-ID: <c1f11f45-bce4-8451-fea8-52ef4c1c8f67@4pi.com>
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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?