course PHY 121
7/19 10
ph1 query 0
Suppose you use a computer timer to time a steel ball 1 inch in diameter rolling down a straight wooden incline about 50 cm long. If the computer timer indicates that on five trials the times of an object down an incline are 2.42sec, 2.56 sec, 2.38 sec, 2.47 sec and 2.31 sec, then to what extent do you think the discrepancies could be explained by each of the following:
• The lack of precision of the TIMER program. To what extent to you think the discrepancies are explained by this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv I think the lack of precision accounts for a large portion of the discrepancies of the exercise, though not quite as much as the human delay of clicking.
• The uncertain precision of human triggering (uncertainty associated with an actual human finger on a computer mouse). To what extent to you think the discrepancies are explained by this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv I think this is the main cause of discrepancies in the exercise; even though the computer may be “off”, the computer’s delay will still be less than that of human reflexes.
• Actual differences in the time required for the object to travel the same distance. To what extent to you think the discrepancies are explained by this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Actual differences in the time the object takes to travel account for a small portion of the discrepancies. While it is likely that the results vary slightly (assuming no other altered conditions), they probably do not deviate as much as shown by the timer.
• Differences in positioning the object prior to release.To what extent to you think the discrepancies are explained by this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Differences in positioning can definitely affect the time it takes for the object to travel. Whether or not the object was completely stationary also makes a difference.
• Human uncertainty in observing exactly when the object reached the end of the incline. To what extent to you think the discrepancies are explained by this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv The eye can process light fairly quickly, but it’s a personal determination of what constitutes completion of the exercise that controls the timer. This estimate or approximation of completion may vary from trial to trial.
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Question: How much uncertainty do you think each of the following would actually contribute to the uncertainty in timing a number of trials for the ball-down-an-incline lab?
• The lack of precision of the TIMER program. To what extent to you think this factor would contribute to the uncertainty?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv This definitely contributes to the uncertainty and will be a factor each time the experiment is carried out.
• The uncertain precision of human triggering (uncertainty associated with an actual human finger on a computer mouse). To what extent to you think this factor would contribute to the uncertainty?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Human triggering also has an effect on the precision of the results, and only adds to the lack of precision of the timer.
• Actual differences in the time required for the object to travel the same distance. To what extent to you think this factor would contribute to the uncertainty?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Depending on the forces at work on the ball, there will be slight variation in the time it takes to travel the distance. In a perfect environment, however, the ball would take the same amount of time every time.
• Differences in positioning the object prior to release. To what extent to you think this factor would contribute to the uncertainty?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Whether or not the ball was completely stationary when released or if it was gently nudged can contribute to differences in the time it takes for the experiment to complete each trial.
• Human uncertainty in observing exactly when the object reached the end of the incline. To what extent to you think this factor would contribute to the uncertainty?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Human uncertainty is definitely a factor; different people will have different ideas of what constitutes the “finish line”.
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Question: What, if anything, could you do about the uncertainty due to each of the following? Address each specifically.
• The lack of precision of the TIMER program. What do you think you could do about the uncertainty due to this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv There isn’t really anything you can do about the TIMER because it is a computer program. The only other option would be to get a better program, if that’s possible.
• The uncertain precision of human triggering (uncertainty associated with an actual human finger on a computer mouse).What do you think you could do about the uncertainty due to this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Ideally, you could train yourself to have slightly better reflexes, but there is a certain degree of delay that you will never be able to erase.
• Actual differences in the time required for the object to travel the same distance. What do you think you could do about the uncertainty due to this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv All you can do to make sure the times are as reasonable as possible is to have the experiment set up exactly the same way every time and make sure that all of the factors are the same (ex – don’t use two different balls because one could weigh more).
• Differences in positioning the object prior to release. What do you think you could do about the uncertainty due to this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv The only thing you can do is try your best to make sure it is set up the same way every single time.
• Human uncertainty in observing exactly when the object reached the end of the incline.What do you think you could do about the uncertainty due to this factor?
your answer: vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv You could possibly give yourself a reference point, such as a line drawn across what you consider to be the “finish” line, which will help but not completely eliminate observation errors.
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Question: If, as in the object-down-an-incline experiment, you know the distance an object rolls down an incline and the time required, explain how you will use this information to find the object 's average speed on the incline.
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Your solution: You can divide the distance by the time to get a rate that will show the “speed” or distance/second.
Confidence rating: 3
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Question: If an object travels 40 centimeters down an incline in 5 seconds then what is its average velocity on the incline? Explain how your answer is connected to your experience.
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Your solution: 40/5 = 8 inches per second. This is similar to the object-down-an-incline experiment.
Confidence rating: 3
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Question: If the same object requires 3 second to reach the halfway point, what is its average velocity on the first half of the incline and what is its average velocity on the second half?
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Your solution: 20/3 = 6.67 inches/second on the first half and 20/2 = 10 in/sec on the second half.
Confidence rating: 3
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Question: According to the results of your introductory pendulum experiment, do you think doubling the length of the pendulum will result in half the frequency (frequency can be thought of as the number of cycles per minute), more than half or less than half?
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Your solution: Based on my results, doubling the length seems to result in more than half the frequency.
Confidence rating: 2
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Question: Note that for a graph of y vs. x, a point on the x axis has y coordinate zero and a point on the y axis has x coordinate zero. In your own words explain why this is so.
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Your solution: A point on the axis doesn’t extend into any of the quadrants of the graph, so will therefore only have one non-zero number in its coordinates to specify its position.
Confidence rating: 2
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Question: On a graph of frequency vs. pendulum length (where frequency is on the vertical axis and length on the horizontal), what would it mean for the graph to intersect the vertical axis (i.e., what would it mean, in terms of the pendulum and its behavior, if the line or curve representing frequency vs. length goes through the vertical axis)? What would this tell you about the length and frequency of the pendulum?
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Your solution: [I think] this would be impossible, because for the vertical access to be crossed it would mean that the pendulum length is zero or negative.
Confidence rating: 2
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Question: `qOn a graph of frequency vs. pendulum length, what would it mean for the graph to intersect the horizontal axis (i.e., what would it mean, in terms of the pendulum and its behavior, if the line or curve representing frequency vs. length goes through the horizontal axis)? What would this tell you about the length and frequency of the pendulum?
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Your solution: To intersect the horizontal access would mean that the frequency is zero at a certain length or that the pendulum has come to rest; the pendulum is not moving.
Confidence rating: 3
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Question: `qIf a ball rolls down between two points with an average velocity of 6 cm / sec, and if it takes 5 sec between the points, then how far apart are the points?
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Your solution: 30cm.
Confidence rating: 3
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Given Solution: On the average the ball moves 6 centimeters every second, so in 5 seconds it will move 30 cm.
The formal calculation goes like this:
• We know that vAve = `ds / `dt, where vAve is ave velocity, `ds is displacement and `dt is the time interval.
• It follows by algebraic rearrangement that `ds = vAve * `dt.
• We are told that vAve = 6 cm / sec and `dt = 5 sec. It therefore follows that
• `ds = 6 cm / sec * 5 sec = 30 (cm / sec) * sec = 30 cm.
The details of the algebraic rearrangement are as follows:
• vAve = `ds / `dt. We multiply both sides of the equation by `dt:
• vAve * `dt = `ds / `dt * `dt. We simplify to obtain
• vAve * `dt = `ds, which we then write as{}`ds = vAve *`dt
Be sure to address anything you do not fully understand in your self-critique.
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Question: `qYou were asked to read the text and some of the problems at the end of the section. Tell your instructor about something in the text you understood up to a point but didn't understand fully. Explain what you did understand, and ask the best question you can about what you didn't understand.
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Your solution: I understood most of the question but sometimes I didn’t know what exactly they were asking for. The wording threw me off, I guess. I tried to read through the problems a few times.
Confidence rating: 2
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Question: Tell your instructor about something in the problems you understand up to a point but don't fully understand. Explain what you did understand, and ask the best question you can about what you didn't understand.
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Your solution: I am having a lot of trouble keeping all the formulas straight, despite my best efforts to organize them. I’m also not exactly sure what I’m supposed to just “know” and what I should be expected to look up.
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