Phy 121
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.
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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.
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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: ->->->->->->->->->->->-> (start in the next line):
I chose the white tape roll in video 4 because it is easiest for me to see. At time 20.562, both the tape and the pendulum are even (the tape is almost hidden by the pendulum) at approximately the 6” mark. At time 20.890. the tape is definitely in front of the pendulum, and is just past the “foot” mark on the measuring tape. At time 21.218, the tape has hit the bottom of the ramp, bounced back, and is laying on its side, while the pendulum has not yet reached the bottom of the ramp.
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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 think position could be measured to approximately the inch mark, mainly due to the measuring tape laying flat instead of standing up for a better vie and the grainy view in the video. Time could be measured much more accurately, because of the large, clear, digital clock in the front of the video and the time being measured to the 0.001 decimal place. Just because of the video quality, I think the time measurement will be much more accurate than the distance measurement…. A digital distance measurement (displayed on the screen next to the clock for example) would make both time and distance more accurate.
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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):
Position and clock time can be used to measure the “speed” or the distance traveled by the tape per second, if the tape travels a further distance in a set time, a second for example, than the second before, the tape could be said to be speeding up. If the tape travels the same distance, say 3 inches, but in less time than the three inches before, the same can be said about the tape 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):
The pendulum is not rolling downhill or just plain speeding up, it speeds up, then reaches the bottom of the swinging arc, then slows as it “fights” gravity to reach the top pf the arch, where it literally stops for a moment, and then returns to do the same in the other direction. The clock could be used to determine speeding up or slowing down on an upswing vs a downswing, but comparing an upswing to a downswing, would not be fair because of other variables involved…ie gravity.
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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):
The pendulum would change speed when it changes positions. The point where the downswing changes to the upswing, or the apex of the swing, is where the speed would begin to slow. Mainly because now the pendulum is fighting gravity. The slowing is obvious, maybe not at first, but eventually, at the top of the upswing the pendulum literally stops. If you’ve ever ridden one of those boat rides that swings back and forth like a pendulum, you know what I’m talking about…. That moment of…. Oh CR*P, here we go again!
Very good description.
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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):
If you can measure the distances accurately enough, the clock can be used to see if the time to cover distance intervals is decreasing, meaning the tape is speeding up, staying the same, meaning the speed of the tape is constant, or increasing, meaning the tape is actually slowing down.
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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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45 minutes or so, but I took a short 5 minute break
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&#Good responses. See my notes and let me know if you have questions. &#
&#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. &#