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course Phys201
6/14@11:56
_1_021#$&*
Phys201
Your 'cq_1_02.1' 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_02.1_labelMessages.txt **
ball starts with velocity 4 cm/sec and ends with a velocity of 10 cm/sec.
• What is your best guess about the ball's average velocity?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
ave vel=`ds/`dt
ave vel=(10-4)/1sec
ave vel= 6 cm/sec
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(10 - 4) / (1 s) = 6 / s, not 6 cm/s.
(10 cm/s - 4 cm/s) / (1 s) = 6 cm/s^2, which is not a velocity. This calculation would correspond to the average rate of change of the velocity, were the velocity to have changed during a 1-second time interval.
It's good that you are trying to apply the rate definition, but it is unnecessary for the current question.
What was the velocity at the beginning of the interval, what was the velocity at the end, and how much did the velocity therefore change.
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&&&& (10cm/sec-4cm/s)/(1s)=6/s-Would this be the correct answer? I am confused by your comment. Was it just that I had the units incorrect?&&&&
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The calculation
(10cm/sec-4cm/s)/(1s)
would represent the rate of change of velocity for an object that accelerates from 4 cm/s to 10 cm/s during a time interval lasting 1 second.
However no time interval was given, so you can't assume a time interval of 1 second. Had a time interval been given, you could have used it in the denominator to calculate the average rate of change of the velocity.
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Note that (cm/s) / s is not equal to /s.
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You need to focus on the question
What was the velocity at the beginning of the interval, what was the velocity at the end, and how much did the velocity therefore change.
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What you are asked to calculate here is the average velocity, not the average rate of change of the velocity.
It is necessary to do the following on any question that asks, in the context of the motion of an object, for a rate of change, an average velocity, or an average acceleration:
First recognize which rate you are asked to calculate, and write down whether it is a rate of change of position with respect to clock time or a rate of change of velocity with respect to clock time.
Write down in words (i.e., 'change in ... / change in ..., where you fill in the ... as appropriate for the numerator and for the denominator) what that rate means.
Then abbreviate what you wrote down as vAve = `ds / `dt, or as aAve = `dv / `dt.
If the definition calls for `ds, write down any quantities related to `ds, including units. Often `ds will simply be given, and it will have units of length (e.g., cm, meters, kilometers, feet, inches, miles etc.). Sometimes you will be given two positions, and you will want to identify them and write them down before writing down your calculation for `ds. If this is the case you will want to be very sure that the units of both positions are compatible and both are units of distance (e.g., they are not compatible, and are not the same, if one is in meters and the other in meters / second; meters / second is not in any case a unit of distance).
If the definition calls for `dv, write down any quantities related to `dv, including units. Sometimes `dv will simply be given, and it will have units of velocity (e.g., cm / sec, meters / sec, kilometers / hr, feet / sec, inches/sec, miles/hr etc.). Sometimes you will be given two velocities, and you will want to identify them and write them down before writing down your calculation for `dv. If this is the case you will want to be very sure that the units of both velocities are compatible and both are units of velocity(e.g., they are not compatible, and are not the same, if one is in meters or meters / second^2 or meters / second / second and the other in meters / second; meters / second is a unit of velocity but none of the others are).
Proceed with the definition, and be sure to do the units throughout, and do them correctly, showing the algebraic steps you took in calculating the units. If your algebraic steps are not correct, I will most likely give you additional notes to clarify, If you don't show the albegraic steps, I won't know exactly what steps you took so I won't know just what to tell you, and I'm likely just to tell you that they are wrong and need to be corrected.
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See if you can apply this procedure, step by step to the present question.
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I am assuming here that the time change would be 1 sec. This information is not directly given. I think this is why the question is asking for your best guess.
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• Without further information, why is this just a guess?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
It is just a guess because we have no information that states if it was traveling at a constant velocity or not. We are also assuming its 1 sec because those values are not given.
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• If it takes 3 seconds to get from the first velocity to the second, then what is your best guess about how far it traveled during that time?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
vAve=(10-4)/3sec
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You haven't used units for all quantities in this calculation.
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&&&&vAve=(10cm/sec-4cm/sec)/ (3sec) aAve=2cm/sec&&&&&
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The calculation you give would be appropriate to aAve, not vAve.
The units in your calculation are correct for the calculation of aAve, but the units of your final result are not.
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You are asked here to calculate how far the object traveled, not its average acceleration. Of course the calculation of average acceleration is equally important, and except for units and some confusion between vAve and aAve you have done that calculation, but it isn't relevant to the calculation of 'how far'.
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The key is to correctly find vAve, then use that result with other given information to find `ds.
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vAve=2 cm/sec
I used the equation vAve=`ds/`dt. Am I using the correct equation?
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You should think more basically, in terms of the definition of velocity as an average rate of change.
That definition does, however, lead directly to the equation you quote.
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• At what average rate did its velocity change with respect to clock time during this interval?
answer/question/discussion: ->->->->->->->->->->->-> (start in the next line):
vAve=`ds/`dt
vAve/`ds=`dt
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You are asked to find the average rate of change of the velocity. Solving this equation for `dt isn't necessary, since you have already, at this stage, been given `dt.
You haven't in any case solved the equation correctly. Whenever you solve an equation you need to detail the steps you used to solve it.
It is, however, good that you know to solve or manipulate equations to find the variables you need.
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2/6=3 cm/sec
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2 and 6 have units, which must be specified in the calculation.
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??? Not certain that I did this correctly. My thought was to manipulate the equation to find `dt.
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You need to analyze this question in terms of the definition of average rate of change.
The question does not involve the average velocity, but the average rate of change of the velocity.
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&&&& (10 cm/sec+4cm/sec)/2 =7cm/sec&&&&
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You don't identify this quantity.
This quantity is the average of the initial and final velocities, and since the acceleration is uniform this is the average velocity.
Now that you have the average velocity, which given quantity do you combine it with to find how far the object moved, what is the specific calculation, and what is your result?
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You need to use units throughout your calculations, for a number of reasons.
You also need to think consistently in terms of the definition of average rate of change. You are on the verge of doing so.
&#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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I've inserted extensive notes, including a step by step procedure for identifying the given quantities and using them in the context of a rate question.
You have a number of good and valid calculations, but you don't always identify them correctly (and you still need work on the calculation and use of units). Hopefully my notes will help you sort everything out.
I'm asking for another revision. This time use #### to identify your insertions, rather than the &&&& asked for in the note below.
&#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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