If your solution to stated problem does not match the given solution, you should self-critique per instructions at

   http://vhcc2.vhcc.edu/dsmith/geninfo/labrynth_created_fall_05/levl1_22/levl2_81/file3_259.htm

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Your solution, attempt at solution.  If you are unable to attempt a solution, give a phrase-by-phrase interpretation of the problem along with a statement of what you do or do not understand about it.  This response should be given, based on the work you did in completing the assignment, before you look at the given solution.

005. 

Question:  `qNote that there are 9 questions in this assignment. 

Question:  `q001.  We see that the water depth vs. clock time system likely behaves in a much more predictable detailed manner than the stock market.  So we will focus for a while on this system.  An accurate graph of the water depth vs. clock time will be a smooth curve.  Does this curve suggest a constantly changing rate of depth change or a constant rate of depth change?  What is in about the curve at a point that tells you the rate of depth change?

Your solution: 

Confidence Assessment:

Given Solution: 

`aThe steepness of the curve is continually changing.  Since it is the slope of the curve then indicates the rate of depth change, the depth vs. clock time curve represents a constantly changing rate of depth change.

Self-critique (if necessary):

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Question:  `q002.  As you will see, or perhaps have already seen, it is possible to represent the behavior of the system by a quadratic function of the form y = a t^2 + b t + c, where y is used to represent depth and t represents clock time.  If we know the precise depths at three different clock times there is a unique quadratic function that fits those three points, in the sense that the graph of this function passes precisely through the three points.  Furthermore if the cylinder and the hole in the bottom are both uniform the quadratic model will predict the depth at all in-between clock times with great accuracy. 

Suppose that another system of the same type has quadratic model y = y(t) = .01 t^2 - 2 t + 90, where y is the depth in cm and t the clock time in seconds.  What are the depths for this system at t = 10, t = 20 and t = 90?

Your solution: 

Confidence Assessment:

Given Solution: 

`aAt t=10 the depth is y(10) = .01(10^2) + 2(10) + 90 = 1 - 20 + 90 = 71, representing a depth of 71 cm. 

At t=20 the depth is y(20) = .01(20^2) - 2(20) + 90 = 4 - 40 + 90 = 54, representing a depth of 54 cm. 

At t=90 the depth is y(90) = .01(90^2) - 2(90) + 90 = 81 - 180 + 90 = -9, representing a depth of -9 cm. 

Self-critique (if necessary):

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Question:  `q003.  For the preceding situation, what are the average rates which the depth changes over each of the two-time intervals?

Your solution: 

Confidence Assessment:

Given Solution: 

`aFrom 71 cm to 54 cm is a change of 54 cm - 71 cm = -17 cm; this change takes place between t = 10 sec and t = 20 sec, so the change in clock time is 20 sec - 10 sec = 10 sec.  The average rate of change between these to clock times is therefore

ave rate = change in depth / change in clock time = -17 cm / 10 sec = -1.7 cm/s.

From 54 cm to -9 cm is a change of -9 cm - 54 cm = -63 cm; this change takes place between t = 20 sec and t = 90 sec, so the change in clock time is 80 sec - 20 sec = 70 sec.  The average rate of change between these to clock times is therefore

ave rate = change in depth / change in clock time = -63 cm / 70 sec = -.9 cm/s.

Self-critique (if necessary):

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Question:  `q004.  What is the average rate at which the depth changes between t = 10 and t = 11, and what is the average rate at which the depth changes between t = 10 and t = 10.1?

Your solution: 

Confidence Assessment:

Given Solution: 

`aAt t=10 the depth is y(10) = .01(10^2) - 2(10) + 90 = 1 - 20 + 90 = 71, representing a depth of 71 cm. 

At t=11 the depth is y(11) = .01(11^2) - 2(11) + 90 = 1.21 - 22 + 90 = 69.21, representing a depth of 69.21 cm. 

The average rate of depth change between t=10 and t = 11 is therefore

change in depth / change in clock time = ( 69.21 - 71) cm / [ (11 - 10) sec ] = -1.79 cm/s. 

At t=10.1 the depth is y(10.1) = .01(10.1^2) - 2(10.1) + 90 = 1.0201 - 20.2 + 90 = 70.8201, representing a depth of 70.8201 cm. 

The average rate of depth change between t=10 and t = 10.1 is therefore

change in depth / change in clock time = ( 70.8201  - 71) cm / [ (10.1 - 10) sec ] = -1.799 cm/s.

We see that for the interval from t = 10 sec to t = 20 sec, then t = 10 s to t = 11 s, then from t = 10 s to t = 10.1 s the progression of average rates is -1.7 cm/s, -1.79 cm/s, -1.799 cm/s.  It is important to note that rounding off could have hidden this progression.  For example if the 70.8201 cm had been rounded off to 70.82 cm, the last result would have been -1.8 cm and the interpretation of the progression would change.  When dealing with small differences it is important not around off too soon.

Self-critique (if necessary):

Self-critique Rating:

Question:  `q005.  What do you think is the precise rate at which depth is changing at the instant t = 10?

Your solution: 

Confidence Assessment:

Given Solution: 

`aThe progression -1.7 cm/s, -1.79 cm/s, -1.799 cm/s corresponds to time intervals of `dt = 10, 1, and .1 sec, with all intervals starting at the instant t = 10 sec.  That is, we have shorter and shorter intervals starting at t = 10 sec.  We therefore expect that the progression might well continue with -1.7999 cm/s, -1.79999 cm/s, etc..  We see that these numbers approach more and more closely to -1.8, and that there is no limit to how closely they approach.  It therefore makes sense that at the instant t = 10, the rate is exactly -1.8.

STUDENT COMMENT:

I don't really understand this even after reading the solution

INSTRUCTOR RESPONSE:

You did some rounding in your solutions up to this point (your solutions were otherwise correct), and didn't get all the 9's in some of the numbers.

Done without rounding, the rates are -1.7 cm/s, -1.79 cm/s and -1.799 cm/s.

These represent average rates over shorter and shorter intervals starting at t = 10 sec.

It appears that these average rates are approaching a limit of -1.8 cm/s, which we therefore take to be the instantaneous rate at t = 10 sec.

Self-critique (if necessary):

Self-critique Rating:

Question:  `q006.  In symbols, what are the depths at clock time t = t1 and at clock time t = t1 + `dt, where `dt is the time interval between the two clock times?

Your solution: 

Confidence Assessment:

Given Solution: 

`aAt clock time t = t1 the depth is y(t1) = .01 t1^2 - 2 t1 + 90 and at clock time t = t1 + `dt the depth is y(t1 + `dt) = .01 (t1 + `dt)^2 - 2 (t1 + `dt) + 90.

Self-critique (if necessary):

Self-critique Rating:

Question:  `q007.  What is the change in depth between these clock times?

Your solution: 

Confidence Assessment:

Given Solution: 

`aThe change in depth is .01 (t1 + `dt)^2 - 2 (t1 + `dt) + 90 - (.01 t1^2 - 2 t1 + 90)

= .01 (t1^2 + 2 t1 `dt + `dt^2)  - 2 t1 - 2 `dt + 90 - (.01 t1^2 - 2 t1 + 90)

= .01 t1^2 + .02 t1 `dt + .01`dt^2  - 2 t1 - 2 `dt + 90 - .01 t1^2 + 2 t1 - 90)

= .02 t1 `dt + - 2 `dt + .01 `dt^2. 

Self-critique (if necessary):

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Question:  `q008.  What is the average rate at which depth changes between these clock time?

Your solution: 

Confidence Assessment:

Given Solution: 

`aThe average rate is

ave rate = change in depth / change in clock time = ( .02 t1 `dt + - 2 `dt + .01 `dt^2 ) / `dt = .02 t1 - 2 + .01 `dt. 

Note that as `dt shrinks to 0 this expression approaches .02 t1 - 2.

STUDENT COMMENT

don’t understand how that the dt in this equation approaches 0 when .02(t1)-2?

INSTRUCTOR RESPONSE

If you divide your previous result

.02 (t1 dt) + - 2 (dt) + .01 (dt^2)

by `dt you get .02 t1 - 2 + .01 * `dt.

The shorter the time interval the smaller `dt will be.

As `dt gets shorter and shorter it approaches 0. This doesn't affect the terms .02 t1 and -2, but it does affect .01 * `dt.
As `dt shrinks to zero, .01 * `dt also shrinks to 0.

The limiting value of our expression, as `dt shrinks to 0, is therefore .02 t1 - 2.

Self-critique (if necessary):

Self-critique Rating:

Question:  `q009.  What is the value of .02 t1 - 2 at t1 = 10 and how is this consistent with preceding results?

Your solution: 

Confidence Assessment:

Given Solution: 

`aAt t1 = 10 we get .02 * 10 - 2 = .2 - 2 = -1.8.  This is the rate we conjectured for t = 10.

Self-critique (if necessary):

Self-critique Rating: