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

I just don't see the use in most average speed problems. Like someone else mentioned, i'd rather go into other areas.
As far as just being d=vt, that may be true. However, how many steps are in this problem? How many steps does it look like are in the problem? What is the goal of the problem? Have you asked this question in conjunction with what is the distance traveled? What is the displacement? If the driver completes an integer number of laps, what is the average /velocity/ (vector value)?
This is all rather contrived, and some may say the added questions don't do much or they do too much. It may clue the students into thinking about the difference btn average speed and average velocity. If that is all you want to check, check both.

Have a good one.
Paul.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Rauber, Joel
Sent: Tuesday, July 30, 2013 12:59 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] [SPAM] Re: The Make-Believe World of Real World Physics

I think one of the useful features of the "average velocity" type questions is the simple act of giving the students practice reading a definition and applying it mathematically. A useful skill to develope. It helps to learn what "delta" of anything means. Another useful skill. And it's all done in situations that require nothing more the 8th grade level mathematics (applications of which most students need to practice). As a bonus the typical, off the cuff, without thinking response, is the wrong answer; which again gives practice at carefully reading and applying definitions. I agree with Anthony, I don't see them as some weird gotcha type trick problem.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Anthony Lapinski
Sent: Tuesday, July 30, 2013 8:37 AM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] [SPAM] Re: The Make-Believe World of Real World Physics

These are not puzzles. They can all be solved using d = vt. Motion isn't all about constant acceleration (and free fall).
These d = vt questions have many interesting real-life situations/applications and are worthwile exercises in problem-solving.

Phys-L@Phys-L.org writes:
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.
_______________________________________________
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_______________________________________________
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_______________________________________________
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--
Kirk Bailey
Never use a big word if a diminutive synonym is as efficacious.
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