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Re: [Phys-L] sequence

Wow!
Thanks for all the feedback on sequence. I think this group is amazing and I am honored to be part of it.
I love how so many people have mentioned the spiral approach. In the usual modeling approach, all the motion is first and forces are next. What I am doing currently has a spiral approach, and as long as we stay based on modeling cycles, I am flexible.
Mind you, these are 9th graders and we have to teach the concepts and math as needed.
With ngss on the horizon, anyone think I am not nuts to believe momentum should be taught between constant velocity and balanced forces? Just simple 1D momentum.
Combines mass and velocity, can be segued into momentum transfer rate (force) and is a conserved quantity ( which will come back again).

Thanks again for all the feedback. Reading your responses is like being a kid and hanging out with rock stars.
Even you, tom pfieffer!

Jim Cibulka
Kirkwood high school
On Mar 20, 2014, at 11:00 AM, phys-l-request@phys-l.org wrote:

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Today's Topics:

1. Re: Sequence (thomaspfeiffer2002@yahoo.com)
2. Re: Sequence (Robert Cohen)
3. Re: Sequence (Philip Keller)
4. Re: Sequence (Marty Weiss)
5. Re: [SPAM] Re: Sequence (Anthony Lapinski)
6. Re: Sequence (Anthony Lapinski)
7. Re: Sequence (Paul Lulai)
8. Re: Sequence (Philip Keller)
9. Re: Sequence (Larry Smith)
10. Re: Sequence (Paul Lulai)
11. Re: Sequence (Rauber, Joel)
12. Re: Sequence (Philip Keller)
13. Re: Sequence (Robert Cohen)
14. Re: Sequence (June Nicholas)
15. Re: multiple inconsistent notions (was: sequence) (John Denker)


----------------------------------------------------------------------

Message: 1
Date: Wed, 19 Mar 2014 13:35:11 -0700 (PDT)
From: "thomaspfeiffer2002@yahoo.com" <thomaspfeiffer2002@yahoo.com>
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID:
<1395261311.13800.YahooMailAndroidMobile@web142302.mail.bf1.yahoo.com>
Content-Type: text/plain; charset=us-ascii

Kinda sounds like should I floss first and then brush or vice versa.



------------------------------

Message: 2
Date: Wed, 19 Mar 2014 22:29:22 +0000
From: Robert Cohen <Robert.Cohen@po-box.esu.edu>
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID:
<F5DEDBE631819941B8D4DCD1B5C645AF0E5D69@msxmb2.admin.esu.edu>
Content-Type: text/plain; charset="us-ascii"

I've gone back and forth in the past and have now settled on forces first.

In particular, I first examine he difference between forces as interactions between objects and the "effect" of those interactions (in terms of changes in motion). Getting students to recognize the difference is important. I start with Newton's 3rd law.

I don't need to go into detail regarding displacement, velocity and acceleration. Indeed, I don't introduce the definition of acceleration at all until after I have finished with displacement and 2-D motion.

The main reason I do this is because my students have trouble with ratios, so starting with rates of change and graphs just overwhelms them. Starting with forces first focuses attention on a single main idea (in this case, that changes in motion are associated with unbalanced forces).

I use my own book for this, which helps.

------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University
East Stroudsburg, PA 18301
________________________________________
From: Phys-l [phys-l-bounces@phys-l.org] on behalf of Bill Nettles [bnettles@uu.edu]
Sent: Tuesday, March 18, 2014 6:31 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sequence

I hope this is AFTER you teach them to do unit conversions well.

I like teaching vector algebra first. Even in two dimensions. Of course, in 9th grade you haven't taught trig to them, have you? If you do kinematics without talking about vectors I believe they are going to develop some bad concepts.

Forces are a good way to introduce vectors early.

I'm trying Jeffrey Schnick's approach this year (editing and converting to LaTeX as I go), and I like it. You have to introduce some quantities by fiat: K=1/2 m v^2, U_g=mgy, etc. but you can do some applicable calculations. But kinematics without vector concepts...yech.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of James Cibulka
Sent: Tuesday, March 18, 2014 3:00 PM
To: phys-l@phys-l.org
Subject: [Phys-L] Sequence

Good day all!

Several teachers at my school have decided they want to teach forces first in
our ninth grade physics course.
I currently teach constant velocity, balanced forces, constant acceleration,
constant net force, energy and finally mechanical waves. One teacher does
motion then forces, and another does forces first, then motion.

We are being told to show up with our arguments why we should follow a
certain sequence. All the headache of college teaching with none of the joy
it seems!
Anyone here have an opinion on sequence? I'd love to hear from you!

Jim Cibulka
Kirkwood high school
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l

------------------------------

Message: 3
Date: Wed, 19 Mar 2014 19:53:33 -0400
From: Philip Keller <pkeller@holmdelschools.org>
To: "Phys-L@phys-l.org" <Phys-L@phys-l.org>
Subject: Re: [Phys-L] Sequence
Message-ID:
<CAMEjEfNDZnD2Mqa9WdpWFobeC-B2vWFUDH5-cAM39Tzf0vGKXQ@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

Just thinking at loud here...

What if you start by defining constant velocity and showing what its
position and velocity graphs look like. Then define constant, non-zero,
positive acceleration and show what its position and velocity graphs look
like. At this point, you have invested maybe 3 class periods if you go
slow.

Then move on to forces, momentum, energy, circular motion, gravitation,
whatever you like, returning to kinematics as later, say before you teach
projectile motion but after you teach vectors.

This way, you would have the vocabulary of kinematics in place but not the
equations. You would get to say things like: when no unbalanced forces act
on an object, its velocity graph looks like this,or like this but never
like that.

It feels to me that you could teach a lot of physics before you needed any
further kinematics treatment.


On Wed, Mar 19, 2014 at 6:29 PM, Robert Cohen
<Robert.Cohen@po-box.esu.edu>wrote:

I've gone back and forth in the past and have now settled on forces first.

In particular, I first examine he difference between forces as
interactions between objects and the "effect" of those interactions (in
terms of changes in motion). Getting students to recognize the difference
is important. I start with Newton's 3rd law.

I don't need to go into detail regarding displacement, velocity and
acceleration. Indeed, I don't introduce the definition of acceleration at
all until after I have finished with displacement and 2-D motion.

The main reason I do this is because my students have trouble with ratios,
so starting with rates of change and graphs just overwhelms them. Starting
with forces first focuses attention on a single main idea (in this case,
that changes in motion are associated with unbalanced forces).

I use my own book for this, which helps.

------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University
East Stroudsburg, PA 18301
________________________________________
From: Phys-l [phys-l-bounces@phys-l.org] on behalf of Bill Nettles [
bnettles@uu.edu]
Sent: Tuesday, March 18, 2014 6:31 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sequence

I hope this is AFTER you teach them to do unit conversions well.

I like teaching vector algebra first. Even in two dimensions. Of course,
in 9th grade you haven't taught trig to them, have you? If you do
kinematics without talking about vectors I believe they are going to
develop some bad concepts.

Forces are a good way to introduce vectors early.

I'm trying Jeffrey Schnick's approach this year (editing and converting to
LaTeX as I go), and I like it. You have to introduce some quantities by
fiat: K=1/2 m v^2, U_g=mgy, etc. but you can do some applicable
calculations. But kinematics without vector concepts...yech.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of James
Cibulka
Sent: Tuesday, March 18, 2014 3:00 PM
To: phys-l@phys-l.org
Subject: [Phys-L] Sequence

Good day all!

Several teachers at my school have decided they want to teach forces
first in
our ninth grade physics course.
I currently teach constant velocity, balanced forces, constant
acceleration,
constant net force, energy and finally mechanical waves. One teacher does
motion then forces, and another does forces first, then motion.

We are being told to show up with our arguments why we should follow a
certain sequence. All the headache of college teaching with none of the
joy
it seems!
Anyone here have an opinion on sequence? I'd love to hear from you!

Jim Cibulka
Kirkwood high school
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l


------------------------------

Message: 4
Date: Wed, 19 Mar 2014 20:22:04 -0400
From: Marty Weiss <martweiss@comcast.net>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sequence
Message-ID: <4D977C33-7758-4C9C-A67D-4A70E30D6AB1@comcast.net>
Content-Type: text/plain; charset=us-ascii

3 class periods? really? usually takes weeks to get those points across.
all those other concepts in a short period of time? months here!

On Mar 19, 2014, at 7:53 PM, Philip Keller wrote:

Just thinking at loud here...

What if you start by defining constant velocity and showing what its
position and velocity graphs look like. Then define constant, non-zero,
positive acceleration and show what its position and velocity graphs look
like. At this point, you have invested maybe 3 class periods if you go
slow.

Then move on to forces, momentum, energy, circular motion, gravitation,
whatever you like, returning to kinematics as later, say before you teach
projectile motion but after you teach vectors.

This way, you would have the vocabulary of kinematics in place but not the
equations. You would get to say things like: when no unbalanced forces act
on an object, its velocity graph looks like this,or like this but never
like that.

It feels to me that you could teach a lot of physics before you needed any
further kinematics treatment.



------------------------------

Message: 5
Date: Wed, 19 Mar 2014 20:27:17 -0400
From: "Anthony Lapinski" <Anthony_Lapinski@pds.org>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] [SPAM] Re: Sequence
Message-ID: <fc.000f547409ed817b3b9aca00839e1829.9ed8188@pds.org>
Content-Type: text/plain; charset=UTF-8

Not quite the same analogy. Sequencing in physics really matters as
concepts are challenging and counterintuitive.

Regarding teeth, flossing first is better as it loosens/removes food
particles before the teeth get brushed/cleaned. Ask your dentist!


Phys-L@Phys-L.org writes:
Kinda sounds like should I floss first and then brush or vice versa.

_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l




------------------------------

Message: 6
Date: Wed, 19 Mar 2014 20:36:46 -0400
From: "Anthony Lapinski" <Anthony_Lapinski@pds.org>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sequence
Message-ID: <fc.000f547409ed81a33b9aca001f57a2c6.9ed81a5@pds.org>
Content-Type: text/plain; charset=UTF-8

I agree, physics concepts take time to learn. But it depends on how you
teach/test. Just math? Or concepts (multiple-choice) too? Makes a big
difference. Also depends on how smart your students are, the pace of the
class, how many topics you teach, etc.

Phys-L@Phys-L.org writes:
3 class periods? really? usually takes weeks to get those points
across.
all those other concepts in a short period of time? months here!

On Mar 19, 2014, at 7:53 PM, Philip Keller wrote:

Just thinking at loud here...

What if you start by defining constant velocity and showing what its
position and velocity graphs look like. Then define constant, non-zero,
positive acceleration and show what its position and velocity graphs
look
like. At this point, you have invested maybe 3 class periods if you go
slow.

Then move on to forces, momentum, energy, circular motion, gravitation,
whatever you like, returning to kinematics as later, say before you
teach
projectile motion but after you teach vectors.

This way, you would have the vocabulary of kinematics in place but not
the
equations. You would get to say things like: when no unbalanced forces
act
on an object, its velocity graph looks like this,or like this but never
like that.

It feels to me that you could teach a lot of physics before you needed
any
further kinematics treatment.

_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l




------------------------------

Message: 7
Date: Thu, 20 Mar 2014 01:02:02 +0000
From: Paul Lulai <plulai@stanthony.k12.mn.us>
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID:
<B1EC32D9C7E46A4BAAC8C22C74B019D56BC94115@EXCHANGE2.stanthony.k12.mn.us>

Content-Type: text/plain; charset="us-ascii"

You can start anywhere you want. What changes is the depth the related content is understood at that time and what we hand wave about at that time.
We are so used to kinematics first that we forget that students are discussing constant velocity but have "no physics understanding"* of why the thing goes at a constant velocity. We follow that with acceleration and the kids have "no physics understanding"* of why the thing accelerates. We teachers / instructors are fine with that because that is what we are used to.
Switching it to Energy First, Forces First, Momentum First, Circuits First, just changes where the hand waving is. Right now we hand wave over the discussion of net force = 0 for constant V and net force = constant for constant a.

Force first and we can hand wave over velocity a bit, but still hit, "what happens to its speed?

I don't do forces first, but I can see a real advantage to starting out with Forces in equilibrium. Structures with Fnet=0 dont change velocity.
How do you measure force? with a scale. What are their units? N or lbs. It says so on the scale. What is a Newton? It is a 1/4 pound. Go to McD's and order a Newton Burger.
All set for forces.

I don't remember who mentioned it, but I really like starting with Newton's third law. We use N3L in TONS & TONS of free body diagrams but hand wave through it.
Why does the car go forward, the person able to walk, the helicopter able to hover, the airplane able to move forward... We use generic terms like thrust. In my opinion, that is missing a HUGE spot to grow their understanding of physics. Do N3L first, and all the fbd can actually be pretty acurate and fictionless (no R).

Have a good one.
Paul.

*no physics understanding because some kids will feel they know what is going on, but they usually are the prior-conceptions that need to be cleaned up.


------------------------------

Message: 8
Date: Wed, 19 Mar 2014 21:17:37 -0400
From: Philip Keller <pkeller@holmdelschools.org>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sequence
Message-ID: <532A41B1.6020107@holmdelschools.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

I spend weeks on this all as well! It may be the thing that Hewitt said
that I most agree with: kinematics can be a black hole right at the
beginning of the course.

But what if the ONLY things you wanted to get across were:

PART I -- here is one kind of motion we care about.
1. When an object moves at a constant speed in a straight line, its
position graph is a line.
2. In that case, the slope of that line stays constant. That slope is
the speed.
3. Since the speed is not changing, the "velocity" graph is a
horizontal line.

PART II--here is another motion we care about.
1. Sometimes, velocity graphs increase or decrease in a linear manner.
2. In that case, speed is changing. The position graphs curve up
(showing increasing speed) or down (showing decreasing speed).
3. The slope of that linear speed graph is called "acceleration".

Suppose we stop right there (for now). No equations. No word
problems. Nothing about area under v vs t. No discussions of positive
vs negative velocity and acceleration. No debates about the
acceleration of an object at its peak. No debates about "deceleration"
vs negative acceleration. No discussion of vectors. All of that can
wait. For now, velocity is just our fancy-pants word for speed.

That's what I think can be done in just a few periods. But again, I was
thinking out loud. I have not tried this approach.



On 3/19/2014 8:22 PM, Marty Weiss wrote:
3 class periods? really? usually takes weeks to get those points across.
all those other concepts in a short period of time? months here!

On Mar 19, 2014, at 7:53 PM, Philip Keller wrote:

Just thinking at loud here...

What if you start by defining constant velocity and showing what its
position and velocity graphs look like. Then define constant, non-zero,
positive acceleration and show what its position and velocity graphs look
like. At this point, you have invested maybe 3 class periods if you go
slow.

Then move on to forces, momentum, energy, circular motion, gravitation,
whatever you like, returning to kinematics as later, say before you teach
projectile motion but after you teach vectors.

This way, you would have the vocabulary of kinematics in place but not the
equations. You would get to say things like: when no unbalanced forces act
on an object, its velocity graph looks like this,or like this but never
like that.

It feels to me that you could teach a lot of physics before you needed any
further kinematics treatment.
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l



------------------------------

Message: 9
Date: Wed, 19 Mar 2014 20:29:19 -0600
From: Larry Smith <larry.smith@snow.edu>
To: <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID: <2467B5C1-3A37-4F6C-BC66-150E5CCE03AC@snow.edu>
Content-Type: text/plain; charset="us-ascii"

I certainly understand (and sympathize with) "you can start anywhere you want," but the real way to answer this question is through physics education research. Does the research say it is equally good to start anywhere you want?

Larry




On Mar 19, 2014, at 7:02 PM, Paul Lulai wrote:

You can start anywhere you want. What changes is the depth the related content is understood at that time and what we hand wave about at that time.
We are so used to kinematics first that we forget that students are discussing constant velocity but have "no physics understanding"* of why the thing goes at a constant velocity. We follow that with acceleration and the kids have "no physics understanding"* of why the thing accelerates. We teachers / instructors are fine with that because that is what we are used to.
Switching it to Energy First, Forces First, Momentum First, Circuits First, just changes where the hand waving is. Right now we hand wave over the discussion of net force = 0 for constant V and net force = constant for constant a.

Force first and we can hand wave over velocity a bit, but still hit, "what happens to its speed?

I don't do forces first, but I can see a real advantage to starting out with Forces in equilibrium. Structures with Fnet=0 dont change velocity.
How do you measure force? with a scale. What are their units? N or lbs. It says so on the scale. What is a Newton? It is a 1/4 pound. Go to McD's and order a Newton Burger.
All set for forces.

I don't remember who mentioned it, but I really like starting with Newton's third law. We use N3L in TONS & TONS of free body diagrams but hand wave through it.
Why does the car go forward, the person able to walk, the helicopter able to hover, the airplane able to move forward... We use generic terms like thrust. In my opinion, that is missing a HUGE spot to grow their understanding of physics. Do N3L first, and all the fbd can actually be pretty acurate and fictionless (no R).

Have a good one.
Paul.

*no physics understanding because some kids will feel they know what is going on, but they usually are the prior-conceptions that need to be cleaned up.
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l



------------------------------

Message: 10
Date: Thu, 20 Mar 2014 02:44:14 +0000
From: Paul Lulai <plulai@stanthony.k12.mn.us>
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID: <exuqejdlyqsu7f8jn4q9hfm2.1395283450221@email.android.com>
Content-Type: text/plain; charset="us-ascii"

I dont believe there is research that has checked every starting point. I was certainly taking some liberties with that statement. Is there research that states kinematics the best place to start? I doubt it.


.:. Sent from a touchscreen .:.
Paul Lulai



-------- Original message --------
From: Larry Smith
Date:03/19/2014 9:35 PM (GMT-06:00)
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sequence

I certainly understand (and sympathize with) "you can start anywhere you want," but the real way to answer this question is through physics education research. Does the research say it is equally good to start anywhere you want?

Larry




On Mar 19, 2014, at 7:02 PM, Paul Lulai wrote:

You can start anywhere you want. What changes is the depth the related content is understood at that time and what we hand wave about at that time.
We are so used to kinematics first that we forget that students are discussing constant velocity but have "no physics understanding"* of why the thing goes at a constant velocity. We follow that with acceleration and the kids have "no physics understanding"* of why the thing accelerates. We teachers / instructors are fine with that because that is what we are used to.
Switching it to Energy First, Forces First, Momentum First, Circuits First, just changes where the hand waving is. Right now we hand wave over the discussion of net force = 0 for constant V and net force = constant for constant a.

Force first and we can hand wave over velocity a bit, but still hit, "what happens to its speed?

I don't do forces first, but I can see a real advantage to starting out with Forces in equilibrium. Structures with Fnet=0 dont change velocity.
How do you measure force? with a scale. What are their units? N or lbs. It says so on the scale. What is a Newton? It is a 1/4 pound. Go to McD's and order a Newton Burger.
All set for forces.

I don't remember who mentioned it, but I really like starting with Newton's third law. We use N3L in TONS & TONS of free body diagrams but hand wave through it.
Why does the car go forward, the person able to walk, the helicopter able to hover, the airplane able to move forward... We use generic terms like thrust. In my opinion, that is missing a HUGE spot to grow their understanding of physics. Do N3L first, and all the fbd can actually be pretty acurate and fictionless (no R).

Have a good one.
Paul.

*no physics understanding because some kids will feel they know what is going on, but they usually are the prior-conceptions that need to be cleaned up.
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l



------------------------------

Message: 11
Date: Thu, 20 Mar 2014 13:31:39 +0000
From: "Rauber, Joel" <Joel.Rauber@SDSTATE.EDU>
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID: <D9FFB2A414E03D40873588FFDDFA5A1F86663CFA@sdsu-ex04>
Content-Type: text/plain; charset="us-ascii"

One nit to pick.

Phillip K wrote in part:


But what if the ONLY things you wanted to get across were:

PART I -- here is one kind of motion we care about.
1. When an object moves at a constant speed in a straight line, its position graph is a line.
2. In that case, the slope of that line stays constant. That slope is the speed.
3. Since the speed is not changing, the "velocity" graph is a horizontal line.

[:]
[:]
The slope is the velocity not the speed. If the object were moving at a constant speed in the opposite direction the slope would be negative, the sign indicating the direction of the velocity vector in a 1D situation (which I'm assuming is what is happening here). Unless you only allow objects to move in the positive direction of how you oriented your coordinate axis.


------------------------------

Message: 12
Date: Thu, 20 Mar 2014 10:08:15 -0400
From: Philip Keller <pkeller@holmdelschools.org>
To: "Phys-L@phys-l.org" <Phys-L@phys-l.org>
Subject: Re: [Phys-L] Sequence
Message-ID:
<CAMEjEfO6QiH6T5TRPmo_to4PQ+RcOakNsoRma0NX3KSEwGc=+A@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

If I were going to try this approach, I would in fact limit motion to
positive velocities. I see the appeal of forces before kinematics. But I
want to be able to refer to constant velocity vs constant acceleration as I
teach about forces. So I would leave this nit unpicked until I came back
to kinematics later.


On Thu, Mar 20, 2014 at 9:31 AM, Rauber, Joel <Joel.Rauber@sdstate.edu>wrote:

One nit to pick.

Phillip K wrote in part:


But what if the ONLY things you wanted to get across were:

PART I -- here is one kind of motion we care about.
1. When an object moves at a constant speed in a straight line, its
position graph is a line.
2. In that case, the slope of that line stays constant. That slope is
the speed.
3. Since the speed is not changing, the "velocity" graph is a horizontal
line.

[:]
[:]
The slope is the velocity not the speed. If the object were moving at a
constant speed in the opposite direction the slope would be negative, the
sign indicating the direction of the velocity vector in a 1D situation
(which I'm assuming is what is happening here). Unless you only allow
objects to move in the positive direction of how you oriented your
coordinate axis.
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l


------------------------------

Message: 13
Date: Thu, 20 Mar 2014 14:17:03 +0000
From: Robert Cohen <Robert.Cohen@po-box.esu.edu>
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID:
<F5DEDBE631819941B8D4DCD1B5C645AF0E5F74@msxmb2.admin.esu.edu>
Content-Type: text/plain; charset="us-ascii"

I used to think that graphs helped students understand the relationship between position, velocity and acceleration I've since learned that students have just as much trouble with graphs as with equations, so introducing graphs at the same time as the relationships does not help them.

It is for this reason that we start with motion. Students can tell when objects are moving. They can't tell if a falling object is speeding up, though. So, we start with objects at rest and discuss how the net force on the object is zero in that case. We then examine what happens to the object, starting at rest, if the net force is not zero. We use a motion sensor to plot the velocity. They see that the velocity increases at a constant rate as long as the net force remains the same.

We do not introduce the word acceleration at this point, as we know they confuse that word with velocity. Besides, it is unnecessary.

We then examine what happens to the object if it is NOT starting at rest. Again, there is no need to introduce acceleration or displacement.

Only after thoroughly examining the relationship between the net force and the CHANGE in velocity (in one dimension, using positive and negative), do we explore how to predict the displacement. We don't derive the kinematic equations. Instead, we have students predict the displacement by first identifying the forces, then identifying the change in velocity (using N2L), then identifying the average velocity (assuming constant forces), then using the definition of average velocity to get the displacement.

After that, we explore 2-D motion.

After that, we introduce momentum, acceleration and work, as three alternate ways of describing what is going and making predictions, comparing the three in terms of their advantages and disadvantages relative to each other.

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Robert A. Cohen, Department of Physics, East Stroudsburg University
East Stroudsburg, PA 18301
________________________________________
From: Phys-l [phys-l-bounces@phys-l.org] on behalf of Philip Keller [pkeller@holmdelschools.org]
Sent: Wednesday, March 19, 2014 9:17 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Sequence

I spend weeks on this all as well! It may be the thing that Hewitt said
that I most agree with: kinematics can be a black hole right at the
beginning of the course.

But what if the ONLY things you wanted to get across were:

PART I -- here is one kind of motion we care about.
1. When an object moves at a constant speed in a straight line, its
position graph is a line.
2. In that case, the slope of that line stays constant. That slope is
the speed.
3. Since the speed is not changing, the "velocity" graph is a
horizontal line.

PART II--here is another motion we care about.
1. Sometimes, velocity graphs increase or decrease in a linear manner.
2. In that case, speed is changing. The position graphs curve up
(showing increasing speed) or down (showing decreasing speed).
3. The slope of that linear speed graph is called "acceleration".

Suppose we stop right there (for now). No equations. No word
problems. Nothing about area under v vs t. No discussions of positive
vs negative velocity and acceleration. No debates about the
acceleration of an object at its peak. No debates about "deceleration"
vs negative acceleration. No discussion of vectors. All of that can
wait. For now, velocity is just our fancy-pants word for speed.

That's what I think can be done in just a few periods. But again, I was
thinking out loud. I have not tried this approach.



On 3/19/2014 8:22 PM, Marty Weiss wrote:
3 class periods? really? usually takes weeks to get those points across.
all those other concepts in a short period of time? months here!

On Mar 19, 2014, at 7:53 PM, Philip Keller wrote:

Just thinking at loud here...

What if you start by defining constant velocity and showing what its
position and velocity graphs look like. Then define constant, non-zero,
positive acceleration and show what its position and velocity graphs look
like. At this point, you have invested maybe 3 class periods if you go
slow.

Then move on to forces, momentum, energy, circular motion, gravitation,
whatever you like, returning to kinematics as later, say before you teach
projectile motion but after you teach vectors.

This way, you would have the vocabulary of kinematics in place but not the
equations. You would get to say things like: when no unbalanced forces act
on an object, its velocity graph looks like this,or like this but never
like that.

It feels to me that you could teach a lot of physics before you needed any
further kinematics treatment.
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Message: 14
Date: Thu, 20 Mar 2014 07:40:40 -0700 (PDT)
From: June Nicholas <somd_stem@yahoo.com>
To: "Phys-L@Phys-L.org" <Phys-L@Phys-L.org>
Subject: Re: [Phys-L] Sequence
Message-ID:
<1395326440.38952.YahooMailNeo@web161805.mail.bf1.yahoo.com>
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Hello all,

Surprised no one mentioned Chabay/Sherwood - they teach momentum first.? I'm raising it here for a discussion point!

Not sure I agree with all of their approaches.? Though I love more utilization of time diagrams to connect to their math courses right off, and have tried to do more in recent years.

Also much emphasis on the Third Law.

Interestingly, a HS I taught at decided a few years ago to flip the order of instruction where "second semester" content was covered in the first semester and vice versa, on the proposition that it would make the math easier.? I dissented, of course (one of many reasons I no longer teach there!).

Here is? a follow-up question:

I would teach all straight-line (including 2D or 3D and projectile) motion first, from kinematics to energy.? Then I would teach circular and rotational motion, as (in my opinion) the essential vocabulary and concepts had been introduced.

However, a lot of textbooks include circular motion as part of the early chapters.

Any thoughts?? I welcome all!

From June Nicholas
A first-time poster and former HS and TYC instructor who hopes to do more soon






On Thursday, March 20, 2014 10:17 AM, Philip Keller <pkeller@holmdelschools.org> wrote:

If I were going to try this approach, I would in fact limit motion to
positive velocities.? I see the appeal of forces before kinematics.? But I
want to be able to refer to constant velocity vs constant acceleration as I
teach about forces.? So I would leave this nit unpicked until I came back
to kinematics later.


On Thu, Mar 20, 2014 at 9:31 AM, Rauber, Joel <Joel.Rauber@sdstate.edu>wrote:

One nit to pick.

Phillip K wrote in part:


But what if the ONLY things you wanted to get across were:

PART I -- here is one kind of motion we care about.
1.? When an object moves at a constant speed in a straight line, its
position graph is a line.
2.? In that case, the slope of that line stays constant.? That slope is
the speed.
3.? Since the speed is not changing, the "velocity" graph is a horizontal
line.

[:]
[:]
The slope is the velocity not the speed.? If the object were moving at a
constant speed in the opposite direction the slope would be negative, the
sign indicating the direction of the velocity vector in a 1D situation
(which I'm assuming is what is happening here).? Unless you only allow
objects to move in the positive direction of how you oriented your
coordinate axis.
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Message: 15
Date: Thu, 20 Mar 2014 08:28:13 -0700
From: John Denker <jsd@av8n.com>
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] multiple inconsistent notions (was: sequence)
Message-ID: <532B090D.5020201@av8n.com>
Content-Type: text/plain; charset=UTF-8

In computer science, there is a set of problems that are
classified as NP-complete. Any such problem is in some
sense just as hard as any other NP-complete problem.
Meanwhile, there are lots of problems that are even
harder than NP.

There is a category of problems I like to call ESP-complete,
because solving the problem by gazing into a crystal ball
works about as well as anything else.


On 03/20/2014 06:31 AM, Rauber, Joel wrote:

PART I -- here is one kind of motion we care about.
1. When an object moves at a constant speed in a straight line, its position graph is a line.
2. In that case, the slope of that line stays constant. That slope is the speed.
3. Since the speed is not changing, the "velocity" graph is a horizontal line.

The slope is the velocity not the speed. If the object were moving at
a constant speed in the opposite direction the slope would be
negative, the sign indicating the direction of the velocity vector in
a 1D situation (which I'm assuming is what is happening here). Unless
you only allow objects to move in the positive direction of how you
oriented your coordinate axis.

I'm not going to disagree with any of that. Everybody
on this lists understands what is being said there, and
knows it's correct.

My point, however, is that there is provably no logical
way for the student to understand that in the introductory
course.

The problem is, there are /at least/ four different notions
of "length" and the student has no way of knowing which one
we want him to use on any given day. Specifically, if we
move along a specified path from A to B, we have
a) The arc length, as measured by an odometer.
b) The chord length, i.e. the distance as the crow
flies from A to B.
c) The displacement pointing from A to B, with
direction as well as length.
d) The projection of the displacement vector onto the
x and/or y and/or z axes of some frame.

To say the same thing mathematically:
a) Arc length = ? |?r|
b) Chord = |? ?r|
c) Displacement = ? ?r
d) Projection = x? ? ? ?r

I insist that all four of these are correct. Each makes
perfect sense in some situation or another. The problem
is, the student has no way of knowing which one is going
to be the "desired" answer on any given day. It's a
guessing game. It's ESP-complete.

I don't want to argue the point, but one could argue
that item (d) has the /least/ connection to the real
fundamental physics. That should be obvious from the
fact that it's frame-dependent, while the others are
not. This just cracks me up, because a huuuge part of
every introductory physics course I've seen emphasizes
viewpoint (d).

Even that wouldn't be so bad if it were consistent, but
in fact the student is expected to use (d) and (a) on
Tuesday and (c) on Wednesday and who-knows-what on Thursday.
This runs counter to every pedagogical principle known to
man. If it were wrong but consistent, the students could
learn it by rote, but if it's inconsistent, even that
doesn't work.

We all do it, all the time, and it drives students nuts,
and you can't blame them.
energy (physics definition versus DoE definition)
conservation (conservative flow, conserve wildlife, conservative force)
acceleration (scalar versus vector)
photon (standing-wave excitation number, running wave packet)
charge (charge on terminal versus gorge on capacitor)
spin (s^2 versus s_z)
adiabatic (isentropic versus isolated)
gravity (framative versus barogenic)
heat (four or five different meanings)
length (at least four different definitions)
time (at least three different definitions)
et cetera......................

The bad news is, it is tremendously hard to notice such
things. The good news is, these are all quite fixable,
once you notice them. You fix them using adjectives
and the like: Odometer distance, proper distance, arc
length, chord distance, crow-flies distance, path of
least action, direction and magnitude, components
and projections .........

Another thing that helps is to point out the problem.
Students are expecting physics to be logical. Alas
the logic can get derailed by various little things.
Even if the teacher managed to be 100% consistent in
the classroom, the students would still need to learn
how to deal with rampant nonsense in the real world.

===========

If I may pick on viewpoint (d) just a little bit more:
They say mathematics is the language of science. That
may be, but it speaks with an accent. Viewpoint (d)
makes the mathematics work great, but it obscures a
great deal of the physics. The grade-school notion
of a vector as having direction and magnitude is far
more elegant, far closer to the real physics ...
but the mathematics of direction and magnitude is a
nightmare.



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