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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.
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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):
I chose the tape and my time/position for each successive frame are: (29.015, 3 in), (29,453, 12 in), (29.671, 20 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: ->->->->->->->->->->->-> (start in the next line):
I would say about 0.1 seconds and 1 inch. We were actually told that the clock was accurate to about 0.01 seconds and looking at the frame by frame we see that from one frame to the next the tape, from our perspective, moves about an inch or is at least between two inch marks. The clock also moves somewhat erratically from frame to frame but the 0.1s digit seems to jump the least of the decimal digits so I think that is the best we can tell from the video. Also the clock and the tape are very fuzzy in some frames so the video quality and frame rate make a big difference. With a high speed camera we could measure much more accurately than we have here (just watch Mythbusters).
I just started watching some of their episodes, when time permits. Good stuff and I enjoy their style.
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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):
Here we would have to use intervals of similar distance and see if the time it takes to cross that distance is the same or not. This could be done with differently sized position intervals but then we would be looking at difference between the average velocities of the intervals not the time differences.
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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):
We could use the same method of above. Using each interval to find velocity and compare velocities to find any changes between them but here we have to take into the account the oscillations because if we aren't careful about the intervals we choose we could receive very deceptive results. For instance, if we measure the time it takes for the pendulum to move from center to one extremity and back then the average velocity will include, but not necessarily reveal, the stop at the apex of the swing and the change in position would have to be measured differently as the net displacement in this example is 0 but the pendulum has travelled a definite distance.
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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 mentioned the difficulty here in my answer above. To do this we could observe the intervals during which we feel the pendulum is slowing down/speeding up and then take measurements over this interval. We would probably need to take relatively small intervals compared to the ones we could use on the tape roll because we need to look at fractional parts of the period of the oscillations.
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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):
Here we could use the average velocities we obtained above and subtract them to find the differences between them. We then compare those differences and determine if they are changing at the same rate with respect to time or not.
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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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It took ±15 minutes.
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Will we ever try to work this out explicitly using the given, or our own, recorded video data?
We can of course record this with much better quality. We'll be taking a video of tomorrow's project, and we'll also do a sound recording.