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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:to a group of non-science majors. This, I have never done before, and it's
For the first time this Fall, I'm teaching a class in "How Things Work"
a bit daunting to know that I need to connect to them in a different way
from STEM majors.
explains that the dishes remain on the table when you whisk a tablecloth
At any rate, we're using the book How Things Work, by Bloomfield, and he
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?
first law, but it seems to me that a glider on an air track or a puck on an
Like I said, I do understand how one could use this demo to discuss the
air table are more instructive given a finite class time. What does
everyone think?
Stefan Jeglinski
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_______________________________________________
Forum for Physics Educators
Phys-l@www.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l