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

["M. Horton" <scitch@verizon.net>]

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
>
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
>
> __________ Information from ESET Smart Security, version of virus 
> signature database 5490 (20100929) __________
>
> The message was checked by ESET Smart Security.
>
> http://www.eset.com


__________ Information from ESET Smart Security, version of virus signature database 5490 (20100929) __________

The message was checked by ESET Smart Security.

http://www.eset.com