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Phy 201
Your 'cq_1_14.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
** CQ_1_14.1_labelMessages **
Seed Question 14-1
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).
• 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.
• 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 using the text editor or word processor. You will then copy-and-paste it into the box below, and submit.
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A rubber band begins exerting a tension force when its length is 8 cm. As it is stretched to a length of 10 cm its tension increases with length, more or less steadily, until at the 10 cm length the tension is 3 Newtons.
• Between the 8 cm and 10 cm length, what are the minimum and maximum tensions, and what do you think is the average tension?
answer/question/discussion: ->->->->->->->->->->->-> :
For the sake of calculation, the minimum tension is 0N at 8 cm and maximum is 3N at 10 cm. That would make the average tension 1.5.
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Good.
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• How much work is required to stretch the rubber band from 8 cm to 10 cm?
answer/question/discussion: ->->->->->->->->->->->-> :
dW = F*ds
dW= 3N*.02m = .06J
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You don't have just one force. You have a force which changes linearly with position, from 0 N to 3 N.
It makes no more sense to use the maximum tension attained over that interval (i.e., the 3 N, which is attained only at the end of the interval) than to use the minimum tension (0 N, which is present only at the very beginning of the interval).
`dW = F_ave * `ds
You specified F_ave in the preceding, and should have used it in the calculation of the work.
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• During the stretching process is the tension force in the direction of motion or opposite to the direction of motion?
answer/question/discussion: ->->->->->->->->->->->-> :
The tension force is in the opposite direction of the motion
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• Does the tension force therefore do positive or negative work?
answer/question/discussion: ->->->->->->->->->->->-> :
The tension force is doing negative work
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The rubber band is released and as it contracts back to its 8 cm length it exerts its tension force on a domino of mass .02 kg, which is initially at rest.
• Again assuming that the tension force is conservative, how much work does the tension force do on the domino?
answer/question/discussion: ->->->->->->->->->->->-> :
I’m not sure. I guess it does .06J of work on the domino
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Assuming that the tension forces are the same on the return as they were on the extension, this would be so (except your result for the work would need to be changed per my preceding note).
On the return the tension force would be in the direction of motion, so would do positive work on the domino.
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• Assuming this is the only force acting on the domino, what will then be its kinetic energy when the rubber band reaches its 8 cm length?
answer/question/discussion: ->->->->->->->->->->->-> :
.06J?
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GIven your result for the work, this would be correct.
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• At this point how fast will the domino be moving?
answer/question/discussion: ->->->->->->->->->->->-> :
.06J = dKE = ½ m*vf^2
.06J= .1*vf^2
6=vf^2
Sqrt(6) = vf
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Your reasoning is good throughout. Had you used the average force to get the work, everything would have worked out correctly.
It shouldn't take you more than a couple of minutes to do a revision.
&#Please see my notes and submit a copy of this document with revisions, comments and/or questions, and mark your insertions with &&&& (please mark each insertion at the beginning and at the end).
Be sure to include the entire document, including my notes. &#
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Since your logic was correct throughout, for your revision you can just edit your document and correct those numbers that need to be corrected. That should be even quicker.
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