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course Phy 121

ph1 query 0Most queries in this course will ask you questions about class notes, readings, text problems and experiments. Since the first two assignments have been lab-related, the first two queries are related to the those exercises. While the remaining queries in this course are in question-answer format, the first two will be in the form of open-ended questions. Interpret these questions and answer them as best you can.

Different first-semester courses address the issues of experimental precision, experimental error, reporting of results and analysis in different ways and at different levels. One purpose of these initial lab exercises is to familiarize your instructor with your work and you with the instructor 's expectations.

Comment on your experience with the three lab exercises you encountered in this assignment or in recent assignments.

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Question: This question, related to the use of the TIMER program in an experimental situation, is posed in terms of a familiar first-semester system.

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:

Are the discrepancies in timing on the order of 0.1 second, 0.01 second, or 0.001 second?

The discrepancies in timing for the above given times are of 0.1 seconds.

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To what extent do you think the discrepancies in the time intervals could be explained by each of the following:

• The lack of precision of the TIMER program. Base your answer on the precision of the TIMER program as you have experienced it. What percent of the discrepancies in timing do you think are due to this factor, and why do you think so?

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The timer program from what I have experienced is very precise, up to the .00001 second in some cases. I don’t think many of the discrepancies come from the use of the TIMER program.

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• The uncertainty associated with human triggering (uncertainty associated with an actual human finger on a computer mouse). What percent of the discrepancies in timing do you think are due to this factor, and why do you think so?

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I would say a lot of the discrepancies come from this. A human is imperfect, and therefore unable to hit the computer mouse at the exact same point in each roll.

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• Actual differences in the time required for the object to travel the same distance. What percent of the discrepancies in timing do you think are due to this factor, and why do you think so?

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The only way I would think this is possible is if the person conducting the experiment happened to hit the computer mouse before the ball started to roll, or vise-versa.

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• Differences in positioning the object prior to release. What percent of the discrepancies in timing do you think are due to this factor, and why do you think so?

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Positioning the ball in different places is a big factor as well. If you position the ball even the slightest bit above or below the initial placement, it will throw your timing off. This would be an easy mistake to make.

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• Human uncertainty in observing exactly when the object reached the end of the incline. What percent of the discrepancies in timing do you think are due to this factor, and why do you think so?

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I would say this also plays a big part, because again, humans are imperfect, therefore not being able to judge the exact same point it reaches the end of the incline each time.

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Question: If you had carefully timed the ball and obtained the results given above, how confident would you be that the mean of those five intervals was within 0.1 seconds of the actual mean? (Note that the mean of the given intervals is 2.43 seconds, as rounded to three significant figures)? Briefly explain your thinking.

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I would be very confident in stating that the mean is in 0.1 seconds of the actual mean because the experiment was carefully timed.

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How confident would you be that the 2.43 second mean is within .01 second? Briefly explain your thinking.

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I would not be very confident in this because .01 seconds is very precise and the accuracy of a human stopping the timer within .01 of that each time is not very likely. #$&*

How confident would you be that the 2.43 second mean is within .03 second?

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Once more, I would not be very confident because a one-hundredth of a second if very precise.

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At what level do you think you can be confident of the various degrees of uncertainty?

• Do you think you could be 90% confident that the 2.43 second mean is within 0.1 second of the actual mean?

• Do you think you could be 90% confident that the 2.43 second mean is within 0.01 second of the actual mean?

• Do you think you could be 90% confident that the 2.43 second mean is within 0.03 second of the actual mean?

Give your three answers and briefly explain your thinking:

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I would be about 90% confident that is in with 0.1 second because it is not too hard to be that precise multiple times.

I could not be 90% confident with the other two because, again, humans are imperfect and being within one one-hundredth or three one-hundredths of a second would require both accuracy and precision.

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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.

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I don’t think anything could be adjusted here.

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• The uncertain precision of human triggering (uncertainty associated with an actual human finger on a computer mouse)

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I can’t think of a way to make a human react any better by themselves.

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• Actual differences in the time required for the object to travel the same distance.

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I cannot think of a way to make this any better.

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• Differences in positioning the object prior to release.

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Make a mark on the incline so that the ball can start at it each time.

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• Human uncertainty in observing exactly when the object reached the end of the incline.

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I cannot think of a way to make this any better.

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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: To solve the objects speed on the incline, you would take distance traveled and divide it by time.

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 cm / 5 sec = 8 cm / sec average velocity.

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 cm / 3 sec = 6.7 cm / sec average velocity for the first half. For the second half, the average velocity is 20 cm / 2 sec = 10 cm / sec.

confidence rating #$&*: 3

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Question: `qAccording 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: Doubling the length of the pendulum will result in more than half of the frequency.

confidence rating #$&*: 3

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Question: `qNote 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:

They each have points on their axis’s labeled zero because these are the x and y intercepts.

confidence rating #$&*: 2

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Question: `qOn 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: If it intersected with the vertical axis, this would mean that the frequency is 0. This would tell you that with change in length of the pendulum, there is also change in frequency.

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: If the graph were to intersect with the horizontal axis, this would mean the pendulum length is 0 cm. If there is no length, there can be no frequency.

confidence rating #$&*: 2

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Question: `qIf a ball rolls 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: If it takes 5 seconds between each point and the velocity is 6 cm/sec, the points are about 30 cm apart.

confidence rating #$&*: 3

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Given Solution:

`aOn 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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Your solution:

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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:

There isn’t anything that I can think of that I had trouble understanding.

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&#Your work looks very good. Let me know if you have any questions. &#