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Re: [Phys-l] First law and balanced forces Was: Re: [PTSOS] anvil andsledgehammer inertia demo

Of course the problem is setting up the force probes to actually measure the
forces. This is not a trivial problem. The friction pads on fan carts are
very small and neither the standard friction pads nor the fan have any
simple attachments to be able to connect them to force probes. The other
difficulty is that if there is the slightest difference, the students will
take it as confirmation of their Aristotelian conception.

But this is close to the standard setup in RTP. They use two fans on the
same cart. One can use a 20ohm pot to vary one of the fans until they are
absolutely balanced. If students see the cart not moving, they know the
fans are balanced. This provides an anchor conception. Then after
predicting they see that the cart after being initially pushed goes at a
constant rate, we have bridged over to the target conception. This has the
advantage that they perceive the forces as being applied to the cart. The
double Atwood distracts students from what you want them to see according to
Ron Thornton. It is easy and relatively cheap.

But notice most physicists tend to come up with example of the trivial case
and not a good convincing example of the target conception. Terminal
velocity, cars at constant speed... are all examples where the student
"knows" there is a net push on the object to maintain the speed. After all
you push on the accelerator to maintain constant speed, and ease off to slow
down. Actually in the terminal velocity case students may tell you the
gravitational force has changed!!! They often believe it is velocity
dependent, needs a rotation, or needs an atmosphere so there is no
gravitational force on the moon!

John M. Clement
Houston, TX

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Bernard Cleyet
Sent: Wednesday, September 29, 2010 5:17 PM
To: Forum for Physics Educators
Cc: NewTeacherWorkshop; David Vernier; Nancy Seese
Subject: [Phys-l] First law and balanced forces Was: Re: [PTSOS] anvil
andsledgehammer inertia demo

An electric powered fanned cart w/ a friction pad would be the automobile
in the room demo. One may vary the power to the fan and the force on the
pad to show constant motion w/ various balanced forces. An ultrasonic
ranger or linearized angular encoder would serve for quantitative
illustration.

bc thinks the balanced Atwood's machine is fine for the cash strapped.

p.s. Often used in HS physics is gee whiz demos, e.g. the anvil on the
tummy. It's unfortunate that such apparently is necessary to keep the
students interested, which is essential for any learning.

On 2010, Sep 29, , at 12:26, ludwik kowalski wrote:


On Sep 29, 2010, at 3:15 PM, Stefan Jeglinski wrote:

The really important demo is to show that a moving object
with balanced forces remains in constant motion.

I substantially agree. What are your favorite demos of this, suitable
for a classroom?

The first thing that came to my mind was driving a car horizontally. I
press the gas pedal too strongly and the car accelerates; I press it much
less strongly and the car slows down.


http://csam.montclair.edu/~kowalski/life/intro.html




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