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 depict the same 'roll' down the ramp, at three different clock times. For each of the three readings, you just 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:
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.)..
answer/question/discussion:
• 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:
• 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:
• 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:
• 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:
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 fine 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 possible 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.
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.
Answer:
I chose the pendulum because it was the easiest to see. I used the first video and the data I collected was: At 59.031, pendulum was released at top of ramp. At 59.687, pendulum reaches the 1 foot mark on tape measure. At 60.125, the pendulum has traveled eight more inches and is now at the 1ft 8in mark on tape measure.
I bet you could give an accurate measure of the time to the nearest .1, but the tape measure is difficult to collect accurate data from, perhaps to the .5in. If the tape measure was closer to the camera, the data would be more accurate.
You can use clock times and object positions to see if an object is speeding up or slowing down by watching how much time it requires the object to move from one place to another. Then you compare two of these times and positions to see if it is traveling faster or slower.
It’s very easy to see if the pendulum is speeding up or slowing down. Take three data points like the three I collected and see how long the pendulum takes to get from point a to b, and then from b to c. The faster the time, the more it’s speeding up and the slower the time the more it is slowing down.
To measure when the pendulum is slowing down, take several data points of position and time. Find the speed the pendulum is moving at these times by dividing in/sec. When the pendulum has a low speed, it is slowing down at that position.
Looking at my data: pendulum travels 12in in .656 seconds then travels 8in in .438 seconds. The first speed is: 18.29 in/sec. The second speed is 18.26 in/sec. The two speeds are almost identical and thus from my experiment, the speed is constant between these data points.
I have a question related to this experiment, from my data, I see that the pendulum’s speed remains constant from the first 12 in and then the next 8in. How could the speed be so constant? Wouldn’t the pendulum be slowing down after the first 12 in or is this point zeroed out by the fact that the pendulum has more momentum after 12in?
The pendulum speeds up until it reaches its equilibrium point, which occurs close to the 12 inch position. Its average speed on one side of the equilibrium position will be equal to its average speed on the other. The motion you observed was not completely symmetric, but was close enough that your average speeds don't differ by much. Given the uncertainties in the data, it is unlikely that the actual speeds are actually as close as your data indicate (a lot depends on the 'luck of the draw').
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I spent approximately 20 minutes on this question.
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Excellent work. See my response to the question you posed at the end.
On future submissions, please insert your answers after the answer/question/discussion: prompt. No problem here, since your answers were all very good, but there will likely be times when it is important for your answers to immediately follow the questions.