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Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo

The Forces topic, in my opinion, is the most conceptually challenging one
in physics.

I couple the terminal velocity coffee filter demo with the blow ball pipe.
For the filters, the weight is initially greater. The air pushing up
balances it, then terminal velocity begins. The filter doensn't hover when
this happens! For the blow ball, the air up is greater until you reach a
steady flow, and then the ball hovers. I give a blow ball pipe to each
student. Also useful when discussing fluids.

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
I agree with you, John Clement. Students will translate the car demo as:

1) A large external force is necessary to accelerate
2) A medium external force is needed to remain at constant speed
3) A small external force is needed to decelerate

This should absolutely not be a first demonstration on this subject.

But a questioning sequence similar to this always worked for me and then
students would understand the car demo afterwards:

Me: "Someone finish this statement . . . An object at rest . . . "
Class: "Stays at rest"
Me: "Anything else?"
Smart Kid: "And an object in motion stays in motion"
Me: "Anything else?"
Crickets: "chirp, chirp, chirp"
Me: "Ok, watch this . . . (roll a ball and it stops). Was
Siiiiiiiiiiiiiiiiirrrrrrrrrr Isaac Newton wrong?"
Class: "Hmm . . . maybe? I don't know? Umm ??? He couldn't be! He's
Isaac Newton!"
Me: "Open your books and see if there was anything missing from that
statement."
Class: "Oooooooooooohhhhhhhhhhhhh, it says 'unless acted upon by an
external
unbalanced force!"
Me: "So what does that mean?"
Crickets: "chirp, chirp, chirp"
Me: "Why did the ball stop rolling?"
Class: "Friction"
Me: "So, what was the external unbalanced force?"
Class: "Friction"
Me: "So, why did the ball stop?"
Class: "Because there were unbalanced external forces on it, in this
case,
friction."
Me: "So, name something that doesn't have unbalanced external forces on
it"
Crickets: "Chirp, chirp, chirp"
Me: "Any object in constant, straight motion or any object at rest has no
unbalanced external forces on it. Name something that doesn't have
unbalanced external forces on it"
Class: "The book on my desk, the computer keyboard, the stack of 250 lab
notebooks waiting to be graded, etc."
Me: "Ok, good. But those were all objects at rest with unbalanced
external
forces on them. Name something that is moving and has no unbalanced
external forces on it"
Crickets: "chirp, chirp, chirp"
Smart kid: "Something in space?"
Me: "Are satellites moving in a straight line? Is the moon moving in a
straight line? Does being in space necessarily mean no external
unbalanced
forces?"
Class: "No. :-( "
Me: "So, name something that is moving that has no unbalanced external
forces on it. Remember anything traveling in a straight line at constant
speed has no unbalanced external forces on it."
Class: "Oh, a car on a straight highway with the cruise control on. A
train
on a straight, level section of track. An object in space that is far
from
anything else."
Me: "Whew! I need to get the crickets out of this room. They're
distracting!"

Is it perfect? Of course not. Will it be criticized by a few list
members?
Of course. Does it work? It did for me. Then, I followed this up with
a
discussion of what the balanced forces are on the car, the train, and the
object in space (or lack of external forces). Then we moved on to
free-body
diagrams with this natural segue.

Mike


----- Original Message -----
From: "John Clement" <clement@hal-pc.org>
To: "'Forum for Physics Educators'" <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, September 29, 2010 12:50 PM
Subject: Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo


Ahh, but the students will interpret this in an Aristotelian fashion.
The
important one is going at constant speed, and they will say that the
push
on
the car is greater than the air resistance to keep it going 60.

Students will not perceive the annular puck demo either. It sounds
good,
but is not very convincing.

There are some demos that work, but the students have to be absolutely
convinced that the forces are non zero and balanced. Come on, if you
read
the research you will know some. You have to start with a situation
where
the students know the answer. The air track with no horizontal forces
is
one where they will say, OK, but it will not attack the balanced forces
example.

And yes there is an answer to this puzzle.

John M. Clement
Houston, TX



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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_______________________________________________
Forum for Physics Educators
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https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l