PHY 231
Your 'cq_1_00.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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This is a 'seed' question. The purpose and the process of answering 'seed'
questions:
In cloud seeding small crystalline particles (the 'seeds') are scattered
throughout a cloud, so that water vapor in the cloud will build up on the 'seed'
and eventually fall in the form of rain.
These questions are posed without any previous explanation. You are expected to
use what you already know, along with common sense, to answer the questions. It
is standard practice in many courses to an instructor to give explanations and
examples before asking students to answer questions, and you will see plenty of
examples and explanations in this course. However the goal here is to first
experience and think about a situation. Whether you think correctly or
incorrectly, your thinking gets you started on an idea and forms a 'seed' on
which understanding can accumulate.
You are expected to answer it to the best of your ability, based on what you
know at the beginning of this assignment.
You are not expected to research this question until after you have submitted
your best response.
You are not penalized based on whether your answer is 'right' or 'wrong', but
you are expected to think as clearly and deeply as you can about the question.
You are not, however, expected to spend hours thinking about the question or
agonize unduly about your answers. A rule of thumb is to give it up to 20
minutes, half for thinking and half for typing in your answers (maybe a little
more for the typing if you don't have good keyboard skills).
This is the very first of the 'seed' questions so this one will probably take
you a bit longer, especially since you have to read all these instructions and
explanations before you get to the actual questions, and also because this one
includes some videos, and because you aren't yet familiar with the process.
Your answers should consist of your best attempt at a solution, and/or one or
more questions about the situation.
If you think you know the answer or can make a reasonable attempt to answer,
then give your answer along with a concise outline of your reasoning.
If you aren't sure what the question is asking, make your best attempt to
interpret and answer it, and consider including one or more questions.
If you are very sure you don't know what the question is asking, then break it
down phrase-by-phrase or even word-by-word and explain what you think each key
phrase or word might mean.
A question consists of a complete but concise statement of what you do and do
not understand about the situation.
There are two ways you can spend an excessive amount of time explaining your
solutions and/or asking questions. One is to type a lot more than what is
necessary, and another is to spend a lot of time worrying about what is and is
not necessary. Balance the two in the way that works best for you.
Remember that the 'concise' part is more for your benefit than mine. I can read
a lot more quickly than you can type, and don't mind reading through a lot of
words to understand your meaning.
You are invited but not required to include comments and/or discussion.
You are welcome to use reasonable abbreviations in your work.
Copy the problem below into a text editor or word processor.
This form accepts only text so a text editor such as Notepad is fine.
Include the text of the entire problem, starting with the words 'The Problem:'.
You might prefer for your own reasons to use a word processor (for example the
formatting features might help you organize your answer and explanations), but
note that formatting will be lost when you submit your work through the form.
If you use a word processor avoid using special characters or symbols, which
would require more of your time to create and will not be represented correctly
by the form.
As you will see within the first few assignments, there is an easily-learned
keyboard-based shorthand that doesn't look quite as pretty as word-processor
symbols, but which gets the job done much more efficiently.
You should enter your answers into this copy using the text editor or word
processor. Enter your response to each question following the
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
prompt.
You will then copy-and-paste the document, which will include the questions and
your answers, into the box below, and submit.
The videos
There are four short videos, all of the same system. The smaller files are
around 500 kB and will download faster than the larger files, which are about 4
times that size (about 2 mB or 2000 kB), but the larger files are a bit better
in quality. If you have a fast connection any of these files should download
fairly quickly. Video 1 and Video 2 probably contain the best information;
Video 4 is the shortest.
The quality of these videos is not that great, and that is deliberate. These
are medium-definition videos, taken with a camera that doesn't have a
particularly high shutter speed. It's not important here to even know what a
shutter speed is, but the effect of the slow shutter speed is to cause images of
moving objects to be blurry.
All data in any science is in effect 'blurry'--there are limits to the precision
of our measurements--and we start off the course with images that have obvious
imperfections. We will later use images made with a high-definition camera with
a fast shutter, where imperfections, though still present, are difficult to
detect.
Video 1 (smaller file) Video 1 (larger file)
Video 2 (smaller file) Video 2 (larger file)
Video 3 (smaller file) Video 3 (larger file)
Video 4 (smaller file) Video 4 (larger file)
View these videos of a white roll of tape rolling down an incline next to a dark
swinging pendulum, using Windows Media Player or a commercial media player. By
alternately clicking the 'play' and 'pause' buttons you will be able to observe
a series of positions and clock times.
The measuring tape in the video may be difficult to read, but it is a standard
measuring tape marked in feet and inches. At the 1-foot mark, a little to the
left of the center of the screen, there is a black mark on the tape. If you
want to read positions but can't read the inches you can count them to the right
and left of this mark. You can estimate fractions of an inch. You don't need
to write anything down; just take a good look.
Begin by forming an opinion of the following questions; for the moment you may
ignore the computer screen in the video. You don't have to write anything down
at this point; just play with the videos for a couple of minutes and see what
you think:
Is the tape speeding up or slowing down?
The tape is speeding up.
Is the pendulum speeding up or slowing down?
The pendulum is speeding up.
Which speeds up faster, the tape or the pendulum?
The tape speeds up faster.
What is going to limit your ability to precisely measure the positions of these
objects?
The video quality.
The computer in the video displays the running 'clock time', which is accurate
to within something like .01 second. Think about how the information on this
screen can help answer the above questions.
You don't have to think about the following right now, so I'm going to make it
easy to ignore by putting it into small type. There is a parallax issue here.
You don't even have to know what this means. But if you do, and if you want the
information, here it is:
The measuring tape is pretty much parallel to the paths of the pendulum and the
tape roll, about 5 inches further from the camera than the path of the pendulum,
and the path of the ball is about halfway between the two. The camera is about
5 feet away from the system.
The problem:
You don't have to actually do so, but it should be clear that if you wished to
do so, you could take several observations of positions and clock times. The
main point here is to think about how you would use that information if you did
go to the trouble of collecting it. However, most students do not answer these
questions in terms of position and clock time information. Some students do not
pause the video as instructed. To be sure you are thinking in terms of
positions and clock times, please take a minute to do the following, which
should not take you more than a couple of minutes:
Pick one of the videos, and write down the position and clock time of one of the
objects, as best you can determine them, in each of three different frames. The
three frames should all depict the same 'roll' down the ramp, i.e. the same
video clip, at three different clock times. They should not include information
from two or more different video clips.
For each of the three readings, simply write down the clock time as it appears
on the computer screen, and the position of the object along the meter stick.
You can choose either object (i.e., either the pendulum or the roll of tape),
but use the same object for all three measurements. Do not go to a lot of
trouble to estimate the position with great accuracy. Just make the best
estimates you can in a couple of minutes.
Which object did you choose and what were the three positions and the three
clock times?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
I chose the tape:
40.140 start
40.469 4in
40.573 7.5in
40.796 12in
#$&*
In the following you don't have to actually do calculations with your actual
data. Simply explain how you would use data of this nature if you had a series
of several position vs. clock time observations:
If you did use observations of positions and clock times from this video, how
accurately do you think you could determine the positions, and how accurately do
you think you would know the clock times? Give a reasonable numerical answer to
this question (e.g., positions within 1 meter, within 2 centimeters, within 3
inches, etc; clock times within 3 seconds, or within .002 seconds, or within .4
seconds, etc.). You should include an explanations of the basis for your
estimate: Why did you make the estimate you did?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
You can only accurately determine it to the nearest inch or half inch of
distance and clock times within the nearest 0.01 second. There were even some
frames where the clock was in the midst of changing so it had an overlay of 2
different times.
#$&*
How can you use observations of position and clock time to determine whether the
tape rolling along an incline is speeding up or slowing down?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
if the distance gets greater over the same amount of time change it is speeding
up
#$&*
How can you use observations of position and clock time to determine whether the
swinging pendulum is speeding up or slowing down?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
Same as the tape.
#$&*
Challenge (University Physics students should attempt answer Challenge
questions; Principles of Physics and General College Physics may do so but it is
optional for these students): It is obvious that a pendulum swinging back and
forth speeds up at times, and slows down at times. How could you determine, by
measuring positions and clock times, at what location a swinging pendulum starts
slowing down?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
Determine at what point the speed, calculated using the change in distance over
the change in time, is less than the previous count. Then you have a ballpark of
where it began to slow down.
#$&*
Challenge (University Physics students should attempt answer Challenge
questions; Principles of Physics and General College Physics may do so but it is
optional for these students): How could you use your observations to determine
whether the rate at which the tape is speeding up is constant, increasing or
decreasing?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
Calculate the change in speed over the change in time.
#$&*
Check to see that you have followed the instructions:
The instructions told you to pause the video multiple times. It appears that
some students are not following this instruction.
If you haven't used the 'pause' and 'play' buttons on your media player, you
should go back and do so.
The questions are phrased to ask not only what you see when you play the video,
but what you see when you pause the video as instructed, and what you think you
could determine if you were to actually take data from the video. You aren't
asked to actually take the data, but you need to answer how you would use it if
you did.
It's OK if you have given more general descriptions, which are certainly
relevant. But answers to the questions should include an explanation of how you
could use the series of position and clock time observations that are may be
observed with this video.
The questions also ask how much uncertainty there would be in the positions and
clock times observable with this specific video. Different people will have
different answers, and some reasonable answers might vary from one clip to the
next, or from one part of a clip to another. However the answers should include
a reasonable quantitative estimate (i.e., numbers to represent the uncertainty;
e.g., .004 seconds of uncertainty in clock times, 2 inches in position
measurements. Use your own estimates; neither of these example values is
necessarily reasonable for this situation). You should also explain the basis
for your estimate: why did you make the estimate you did?
You should have estimated the number of seconds or fraction of a second to
within which you think the time displayed on the computer screen might be
accurate (e.g., is it accurate to within 10 seconds of the actual clock time, or
to within 1 second, within .1 second, maybe even within .01 or .001 second).
You might not yet know enough about the TIMER to give an accurate answer, but
give the best answer you can.
You should also indicate a reasonable estimate of the number of inches or
fraction of an inch to within which you could, if asked, determine the position
of each object.
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20 minutes.
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&#Your work is mostly correct, and I believe you will understand everything after reading the document in the link below. You will be directed to submit a revision; however unless you have questions or comments, the revision is not necessary. Just be sure you understand all the important details in the document. &#