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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.
** CQ_1_00.1_labelMessages **
As on all forms, be sure you have your data backed up in another document, and in your lab notebook .
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?
Is the pendulum speeding up or slowing down?
Which speeds up faster, the tape or the pendulum?
What is going to limit your ability to precisely measure the positions of these objects?
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: ->->->->->->->->->->->-> scussion (start in the next line):
Video 3, observing the tape:
at 29.125 s, the front of the tape appears to be at about 1 ft. 8 inches
at 29.343 s, the front of the tape appears to be at 1 ft. 1 in
at 29.671 s, the front of the tape is in contact with the base of the tape measure, but hasn't fallen over yet, so I'll use that for the time the tape reaches 0 in.
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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: ->->->->->->->->->->->-> scussion (start in the next line):
Stopping the video means we have a pretty exact clock time, so the time would be accurate within .01 seconds as you suggested we should assume the timer to be. I'm really not sure how I can analyze the accuracy of this otherwise.
As for position, that is complicated by the fact that the pendulum is in the way for most of the tape's path, so I am estimating two things, in some cases: where the front of the tape actually would appear if the pendulum weren't there (based on looking at the back of it and estimating size), and also what the tape measure reads at that point. So I think my position estimates would only be accurate within a half an inch.
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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: ->->->->->->->->->->->-> scussion (start in the next line):
Using three time/position points could give us a general idea of whether the object is accelerating. We can find average velocity for two portions of the tape's path by finding the distance between the first and second points I estimated (and then the second and third) and dividing it by the elapsed time between the moments I recorded the tape being at those points.
Then we could compare the two. If the second portion has a greater average velocity, the object has accelerated.
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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: ->->->->->->->->->->->-> scussion (start in the next line):
I suspect, actually, that a pendulum does both, two times each, during the course of one cycle. So this would require collecting quite a bit more data.
To prove that a pendulum speeds up between the moment of being release at one end of its arc and the midpoint of its cycle at the bottom of the arc, we'd need to get three data points: one at the moment of release, one halfway to the midpoint, and one at the midpoint. Then we'd have to compare the first half of the downward path and the second half of the downward path, similar to what we did with the rolling tape. Then we'd have to do the reverse for the upward path from the midpoint to the opposite end of its arc: three data points, at the midpoint, then halfway back up to the opposite extreme, then at the moment it reaches the opposite extreme endpoint.
Of course, you might also mean that each successive cycle of the pendulum might get faster or slower than the last. That would be a bit simpler to study; just stop the clock each time the pendulum gets back to the initial release point, and compare each back-and-forth cycle to see if the elapsed time is changing.
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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: ->->->->->->->->->->->-> scussion (start in the next line):
Well, I suppose I've been assuming that the pendulum speeds up on the way down, due to gravity, and slows down on the way up (because gravity is acting in the opposite direction, even though the momentum from the initial downward pull is still propelling the pendulum in the direction it started out). Of course, the path appears mostly horizontal. But we know it's actually a curved arc because the string holding it stays the same length throughout each swing. The top of the string is basically the midpoint of a circle, and the string is the radius. There is only one moment when the string is exactly plumb, or vertical, and that should be the midpoint of the pendulum's path from one end to the other. Until that moment, and after it, the pendulum is moving from side to side, yes, but it is also moving up or down, however slightly.
So, as I described above, we'd need to collect at least 5 time/position points so we can compare four parts of the pendulum's path (2 down, and 2 up) to verify that it is indeed accelerating and decelerating as expected.
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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: ->->->->->->->->->->->-> scussion (start in the next line):
I believe I'd have to collect more data to be sure of this. As it is, I can only compare two portions of the tape's downward roll-- just enough to determine that it is in fact accelerating. But that can't tell me how much, or at what rate.
I'd say we'd need to collect at least 4 position/time points for this one, too, so that we can have 4 segments' average velocity to compare to each other (a to b, b to c, c to d).
I'm going to pretend we're dealing with much larger time and distance for the sake of using simpler units. If the average speed in section a to b is 1 m/s and the average speed from b to c is 2 m/s, we know it's accelerating. Then we need to look at c to d. If it's 3 m/s, it's accelerating at a constant rate of 1 m/s. But if it's more than that, it's accelerating at an increasing rate.
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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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30 minutes
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Very good.
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