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Phy 242
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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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):
The pendulum
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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):
I could probably estimate the position within 2 inches of the actual position. The total distance of the incline is around 2 feet so I don't think it would be that hard to determine a position within two inches of the actual position since the incline isn't super long. I could probably estimate the clock times within 1 second of the actual clock time because since the
videos themselves are fairly short, there is less room for error when estimating the time.
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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):
To determine if the tape is speeding up or slowing down, I would use three observations of
position and clock time. I would use the first two observed positions and subtract the smaller
distance from the larger distance. This gives me the change in distance between the two points.
I would also subtract the smaller observed time from the larger observed time. This gives me the
change in time between the two points. Then, I would divide my change in distance by my change
in time. This is my 'average velocity' between those two points. I would repeat these exact
steps again except now for the second and third observations. I should now have two different
average velocities. One is the average velocity between points one and two, the other is the
average velocity between points two and three. To determine if the tape rolling is speeding up
or slowing down I would compare the two velocities and see which one is larger and which is
smaller. If the first average velocity is larger, and the second is smaller, this would suggest
that the rolling tape was slowing down. If the first velocity was smaller and the second
velocity was larger, this would suggest that the tape was speeding up.
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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):
Very similar to above, I would start by using three observations of position and clock time. I
would use the first two observed positions and subtract the smaller distance from the larger
distance. This gives me the change in distance between the first two points. I would also
subtract the smaller observed time from the larger observed time. This gives me the change in
time between the first two points. Then, I would divide my change in distance between the two
points by the change in time. This is my 'average velocity' between the first two points. I
would repeat these steps again except now for the second and third observations. After that, I
will have two average velocities, one between the first and second points and the other between
the second and third points. If the velocity between the first two points is greater than the
velocity between the second and third points, this suggests that the pendulum is slowing down.
If the velocity between the first two points is smaller than the velocity between the second and
third points, this suggests that the pendulum is speeding up.
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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):
I think you could determine this location by observing the total distance the pendulum travels
and dividing that distance by two. This gives the midpoint of the distance travelled by the
pendulum. I think the point that it starts slowing down would be somewhere near this point. To
check my guess, I would observe a position and clock time before the point and one after the
point. I would then do what I did in the previous problem and find the two average velocities
between these three points. I would expect that the average velocity between the point before
the mid-distance and the mid-distance would be greater than the average velocity between the
mid-distance and the point after the mid-distance. This would not assure that my mid-distance
value was 100% correct but it would mean that somewhere not far from this point, the slow down
occurs.
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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):
I don't think the use of my observations are necessary to determine this. I know that the rate at which the tape is speeding up is constant because I know it's accelerating due to gravity and gravity is constant.
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That is an assumption. The question asks how you would validate this assumption with experimental evidence.
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I'm not too sure how I would use my observations to determine this. Perhaps by finding several average velocities and observing some sort of pattern or trend between them?
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This is a promising idea. You'll want to think about exactly what you would do with the average velocities, and perhaps other information, to fully answer the question.
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&#This looks good. See my notes. Let me know if you have any questions. &#