Re: Newton's Second Law Lab

[Rick Tarara <rbtarara@SPRYNET.COM>]

I don't know how well this will work in terms of getting a nice linear
relationship, but it might be fun to try.

Find a flat region of your parking lot.  'Borrow' two cars, one very small
and light (Geo Metro or the like) and the other much larger (a big SUV for
example).  Measure off a known distance -- maybe 20 meters or so.  Start
with the small car and one person pushing.  Have them start at rest (car in
neutral) and push as hard as possible until the car moves the measured
distance.  Use a = 2D/t^2 to get the acceleration.  Now repeat, adding
pushers.  Then repeat with the larger vehicle.

I suspect 2 pushers won't achieve twice the acceleration, but surely a
higher acceleration and a given number of pushers will get a lower
acceleration for the large car versus the small.  You need measurers,
timers, and pushers, so you can get most of the class involved.

It is possible to do a little more with the air track experiment, but it
might not be appropriate for your HS class.  I have Ealing tracks with a
nice 'air' pulley at one end where you can use recording tape as the
'massless' rope over the 'frictionless' pulley.  Using photogate timers with
one gate just in front of the stationary cart, we time the motion over a
given distance to yield accelerations.  First we vary the falling mass by
moving mass (slotted masses) from the air cart to the mass hanger.  This
keeps the mass of the system constant.  We only assume that the force is
proportional to the falling mass but act as though we don't know the
proportionality constant.  Then different groups study the acceleration for
a fixed force as the mass of the cart is varied.  Each of these groups
produces an a versus 1/M plot (after seeing that a versus M is not linear).
Each of these plots have different slopes but complex units.  However, at
this point I ask what should we do with this data--different slopes for
different fixed forces (fixed falling mass).  Someone will offer--'We've
plotted everything else in the world, why not plot that.'  When we plot
these slopes versus the fixed falling mass, the relationship is again
linear.  The SLOPE of this plot ends up with units of acceleration and the
magnitude is pretty close to the acceleration due to gravity.  If we then
use this 'physical' constant as the proportionality constant in the first
part of the experiment (plotted as falling mass versus acceleration) the
slopes end up having the units and magnitude of the total mass of the
accelerating system.  There's a little hand-waving here, but it doesn't seem
to the students to be just a 'simple confirm the equation' experiment.  This
one is more mathematical, the car experiment certainly more conceptual.


Rick

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Richard W. Tarara
Associate Professor of Physics
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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----- Original Message -----
From: "David Abineri" <dabineri@CHOICE.NET>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, July 03, 2000 8:12 AM
Subject: Newton's Second Law Lab


> I am trying to come up with a good lab for high school students to do
> that will clarify and test N2.  I have air tracks available but have the
> following experiences.
>
> 1. With a level air track, using a hanging mass as the constant force
> pulling the mass on the track via a pulley seems very complicated for a
> student seeing this kind of thing for the first time. The analysis of
> such a system already requires a knowledge and acceptance of N2 in order
> to figure the force being exerted on the horizontally moving mass so it
> seems circular to the students. It is difficult to measure the force
> directly since the air track can support limited weight.
>
> 2. Last year I tried tipping the air track and measuring the angle of
> tip I could know the force acting down the incline and measured
> acceleration with a motion detector (CBL).  The results were not good
> perhaps because it was difficult to measure the angle well.
>
> Are there other ways I should consider having them check and appreciate
> N2 or, how might I modify the above for a worthwhile lab.  Any ideas are
> welcome.
>
>      Dave Abineri
> --
>  David Abineri   dabineri@choice.net