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Re: [Phys-L] [SPAM] Re: The Make-Believe World of Real World Physics



I think there is real value in getting students to use graphs to understand situations and solve problems. But first they have to understand graphs, and I don't think enough time is spent in math or physics on that. Do students understand what the points on the graph represent, do they understand what horizontal and vertical intervals represent? How do we know they do understand.
These graph solutions can be done with constant velocity problems first and then advanced to more complicated situations as the student understanding develops.
Average velocity also has extra meaning when you look at graphs and begin to talk about slope. I suspect your concern has more to do with the abstract nature of the way the problem is presented, rather than about the content. What say you?

joe
On Jul 30, 2013, at 8:44 AM, Philip Keller wrote:

I agree. These are nice math puzzles but they barely feel like physics to
me. They have the "if Mary can mow a lawn in 40 minutes..." feel to them.
I see this particular type of item as part of the black hole of
kinematics. I would much rather skip this and spend that time trying to
get my students to use graphs to solve problems rather than formulas.


On Mon, Jul 29, 2013 at 9:40 PM, Paul Lulai <plulai@stanthony.k12.mn.us>wrote:

I think this question (how fast / far to have a certain avg speed) is a
lot of work that isnt worth the payoff and is also a bit of a trap.
It feels like we are baiting them into trying to do it incorrectly.
I am sure there are science & technical folks that do a lot with average
speed, but we do next to nothing with it in high school, except ask trick
questions like this one. I suppose it is a way to check the difference btn
speed and the vector nature of velocity. It just seems like there would be
other ways that could check the same conceptual point and be less
misleading.
I don't do much at all w avg speed. While I am sure it can be helpful in
some situations, this isnt an area I am going to go crazy preparing for.

Paul

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




-------- Original message --------
From: Anthony Lapinski <Anthony_Lapinski@pds.org>
Date: 07/29/2013 6:51 PM (GMT-06:00)
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] [SPAM] Re: The Make-Believe World of Real World
Physics


Makes sense.

Acceleration is hard for kids. I begin the course with speed, average
speed, velocity, etc. Another "basic" problem/idea kids in my class get
wrong is about average speed:

Must average 200 mph over 2 laps. After one lap, you've only averaged 150
mph. What must be your average speed for the second lap?

Many want to say 250 mph as they want to just average the speeds. They
forget the definition that avg speed = distance/time. Even after doing
problems in class and for homework, they continue to miss this question
(and variations of it) on the test. Physics requires a different way of
thinking about the world.

You can simplify this with a related question:

On a trip to New York. One way is 60 miles. 60 mph going there and 30 mph
coming back. Is the avg speed for the entire trip less than equal to, or
more than 45 mph?

It's a great peer instruction question as the math is relatively easy to
do in your head, and those that understand the problem can explain it to
others. Still challenging for some. Very interesting discussions to hear
in the classroom!

So do others teachers have similar experiences with these types of average
speed problems?


Phys-L@Phys-L.org writes:
I can certainly envision lots of students claiming the acceleration at
the peak is zero. However, i think this answer comes from students
trying to memorize physics points rather than think about and analyze
physics. In my experience, students that answer a=0 remember there is
something unique about the top. They remember something is zero and they
go with a =0 since we are asking them about a.
If we ask them if there velocity at the peak will remain constant, few
will say /yes the velocity will be 0 and will stay 0 at the top forever
and the ball will never fall/.
Our challenge is to get them to analyze the scenario and avoid the
temptation to answer reflexively.
In my opinion, that is the challenge w a lot of the challenging and
trick/tricky physics questions students encounter in their first course.

Have a good one.
Paul.

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




-------- Original message --------
From: Kirk Bailey <kirkbaile@gmail.com>
Date: 07/29/2013 5:01 PM (GMT-06:00)
To: Phys-L@phys-l.org
Subject: Re: [Phys-L] The Make-Believe World of Real World Physics


I can't remember where or when, but I know I read that the number one
misconception (based on how many students still missed it at the end of
the
course) was the acceleration of an object thrown straight up at the peak
of
its trajectory. Just understanding that it isn't zero at the top is a
non-trivial objective, and I don't see it as an easy question in any
student universe.


On Mon, Jul 29, 2013 at 4:19 PM, Anthony Lapinski
<Anthony_Lapinski@pds.org>wrote:

Right. And a student in a physics class should answer it differently
(correctly). Acceleration = rate of change of velocity = gravity =
constant (in free fall).

Unless, of course, I am missing something or in a different universe.


Phys-L@Phys-L.org writes:
Folks on the street indeed are likely to think of acceleration =
speeding
up and deceleration = slowing down. therefore acc at top = zero.

Every beginning physics student should be able to think of the velocity
graph with it's slope and areas included.

On Jul 29, 2013, at 3:36 PM, John Denker <jsd@av8n.com> wrote:

You might be a redneck physicist if you buy 48 cans of
Big Flats beer and cool them off using liquid nitrogen.


On 07/29/2013 07:29 AM, William Maddox wrote:
In this universe the acceleration of a ball at peak question would
not be considered ill posed in the context of a physics test
following chapters on projectile motion and gravity.

Congratulations on living in such a nice universe (A).

Meanwhile, there are plenty of people on this list who
live in a different universe (B), where students find
this question hard or at least counterintuitive.

I reckon this list is extremely valuable, because it allows
us to recognize and discuss the difference between these
two universes.
-- Why is this an easy question in one universe but not the other?
-- Can we move everybody from universe (B) to universe (A)?
-- If so, how?

I remind everybody yet again that many things that seem
hard at the beginning of the road seem easy (and are easy)
at the end of the road.

=============================

Physicists tend to write as little as they feel necessary.

... which can be a big part of the problem, if they misjudge
what is "necessary". This is known to be a problem whenever
experts are talking to non-experts, including students.
Miscommunication makes things hard, even things that ideally
"should" not be hard.
_______________________________________________
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




--
Kirk Bailey
Never use a big word if a diminutive synonym is as efficacious.
_______________________________________________
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
_______________________________________________
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

Joseph J. Bellina, Jr. Ph.D.
Emeritus Professor of Physics
Co-Director
Northern Indiana Science, Mathematics, and Engineering Collaborative
574-276-8294
inquirybellina@comcast.net