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Re: [Phys-l] Newton and Snell (was Global evolution as fact)



Average in space or average in time?

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of Anthony Lapinski
Sent: Wed 1/12/2011 6:54 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Newton and Snell (was Global evolution as fact)



Interesting thread! I teach in high school and traditionally do mechanics
first. When kids think of physics, they often talk about "motion."
Students have many misconceptions about velocity and acceleration, and
have much difficulty with graphs, slopes, etc. Also, even "basic" math
they get wrong. For example, drive somewhere with an average speed of 30
mph, and return the same way with an average speed of 60 mph. What the
average speed for the entire trip? Turn out not to be 45 mph, against the
"common sense" of even my brightest students.

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
There are some advantages to working with the kinematics equations
despite
all the negatives given by Bob.

1) Students have (often faulty) concepts of velocity and acceleration
that
may need serious work--even if you want to start with momentum and energy.

2) The kinematics equations can be derived easily from basic
definitions.
Sometimes students get the idea that all our equations are just pulled
from
some body orifice! ;-)

3) The basic kinematics equations can be used to build complex solutions
through numerical methods (spreadsheets) where key quantities ARE held
constant over a short period of time to approximate the evolution of a
system. This can give insight into the ultimate meaning differential
equations or at least why we like to write equations as such. The
problem
we work on is a bowling ball falling from 100 km above the earth's
surface
(falls out of the shuttle on the way to orbit). Finding the time to hit
the
ground and the speed at which it hits is the goal--starting with just the
consideration of the changing 'g', then adding in a velocity dependent
air
resistance term, and finally modelling the air resistance with functional
representation of air density with height. Later we use that same
spreadsheet--drop foam balls from various (and some quite large) heights,
and fitting the data with the numerical model. The heart of the
spreadsheet is just that acceleration is a function of height and
velocity,
but for each short time period (.1 seconds typically) the velocity will
change by acceleration x delta-t with the acceleration held constant and
the
height will change by v x delta-t, both relationships straight from the
kinematics equations.

Even if you do work with momentum and energy first, I'm sure you run into
the same problems of students not understanding 'CHANGE IN'. No matter
how
simple it seems to us, that a changing velocity means the object sped up,
slowed down, and/or changed direction and a change in speed/kinetic
energy
means the object sped up or slowed down , these are very difficult ideas
for
many students. Using the most common form of the work-energy theorem
(Net
Work = change in KE) a series of questions asking students to determine
if
the net work is positive, negative, or zero giving a whole list of
actions
(my favorites being parts of the motion of a vertically thrown ball)
remain
mysterious to many, many students. Knowing that this is a major
stumbling
block, I personally like to work on velocity and acceleration separately,
right at the start of the course, and keep working on these throughout
the
course. While you might avoid the word acceleration in a momentum/energy
first approach you still end up dealing with changes in velocity.

Bottom line--I don't see anything wrong with momentum/energy first, but I
don't necessarily see a big advantage either. I also feel there is (can
be)
considerable value in working with the kinematics equations (if nothing
else
as the gateway to 'PHYSICS' problem solving--and this is physics IMO, not
just math.)

Rick

Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
*******************************************
Free Physics Instructional Software
www.saintmarys.edu/~rtarara/software.html
'Hi def' versions being posted as completed.
********************************************



----- Original Message -----
From: "LaMontagne, Bob" <RLAMONT@providence.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Wednesday, January 12, 2011 1:07 PM
Subject: Re: [Phys-l] Newton and Snell (was Global evolution as fact)


John,

I have no doubts that physics can teach these topics more effectively,
especially if there is some hands-on component to the teaching. My
question really goes to whether we should be teaching kinematics at
all.
Most accelerations in practice are not constant. Most projectile
motions
are not parabolic (even in the flat earth approximation) because of
air
resistance. There are many useful topics we choose to leave out of
general
physics. I just hope we are not spending an inordinate amount of time
on
idealized motions just to produce practice problems for F=ma. Most
motion
problems can be done more easily using momentum and energy - and don't
require the artificial restriction of constant acceleration - although
that class of problems is easily included. Take the case of dropping a
ball---most of the interesting parameters of the motion come from
energy
balance - the kinematic equations aren't actually used at all.

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu
[phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
[clement@hal-pc.org]
Sent: Wednesday, January 12, 2011 12:19 AM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] Newton and Snell (was Global evolution as fact)

The big reason for spending time on kinematics is that math does not
teach
them in a manner so as to generate understanding. The research shows
that
students have to use multiple representations: pictorial (motion maps),
graphs, descriptions, and lastly equations. Unfortunately most
conventional
physics courses also do not generate good understanding.

The work of Jerry Epstein has shown that students come out of math
courses
with very low understanding, so we have to teach them the relevant
math.
I
can confirm this from what I have observed in the calculus based
courses.
Actually well over 30 years ago I taught a conceptual physics course at
Duke
U where I stressed kinematic graphs, and a student who had taken
calculus
told me that it made her understand calculus. In addition the students
did
very well at relating velocity, position, and acceleration graphs.

It has been stated that the sequence of starting with impulse momentum
is
superior to the conventional one. I would say that this would need to
be
demonstrated by using conceptual tests such as the FCI/FMCE. At
present
the
Modeling program has some classes that have achieved normalized gain as
high
as 90%, so it would seem to be that the conventional sequence works very
well provided research based instruction is used. So I must take
exception
to the claim that impulse-momentum is superior. It has not been
demonstrated.

Actually the dividing line between math and physics is fairly broad, as
are
all dividing lines between fields. Kinematics is both math and physics.

John M. Clement
Houston, TX


May I also chime in with a preference for your outline of topics. We
spend
too much time with the kinematic equations for constant acceleration -
mostly because we wish to apply F=ma at some point. These are really
math
equations, not physics per se. It may be nice to know how long a runway
must be if an aircraft with a certain acceleration needs a certain take
off speed - but these are math problems. We probably only do them in
our
courses because we spent time developing the kinematic equations. If we
covered other topics, and used impulse<-->momentum instead of F=ma, we
would probably find a whole new set of problems that we would feel are
important.



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

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


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