Re: [Phys-l] Newton's First law Demo

[Michael Porter <listmoe@gmail.com>]

On 2010-03-18, at 6:26 PM, John Clement wrote:
> The first half is genuine, and actually demonstrates the second law, but not
> the first because the objects actually move a bit.

I always present the tablecloth demo as a second law example (where the second law is presented in the form F_net = ∆p/∆t, or impulse = F∆t). There's a force, N#1 does not apply.

> It is cute, but not very useful pedagogically.

I beg to differ -- I showed the video to my class, and challenged them to prove that it must be a fake based on the apparent movement of objects on the table. We figured out how the final speed of an object related to time in contact with the cloth, and how distance travelled over the bare table before stopping related to that speed. The objects at the motorcycle end of the table should move significantly farther than the objects at the near end -- something like 100x farther, based on some admittedly crude calculations from counting frames and estimating lengths. The kids ate it up. We also did a similar analysis of the likelihood of success in the "Megawoosh" video earlier in the year: <http://www.youtube.com/watch?v=lkwh4ZaxHIA>

Mike

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Michael Porter
Colonel By Secondary School
Ottawa, Ontario, Canada