You are missing a multiplication between the end of the sine function and the term that is squared.

When I teach Maple, I make sure that students learn to explicitly include the "*" symbol at all times. For example:

int(sin(n*Pi*x/L)*((x/L)^2)^2, x=0..L)

So you are not alone in making this error. Sometimes it is simply the need for another pair of eyes.

@Christopher2222  You are not alone. My only post ended up listed as a question. Hence I'm not sure what the "post" button does.

Via Google, I had no difficulty locationg your posted on Numb3rs, Maple, etc. posting back in 2009. However, I agree, it does not come up in a search using the search bar above.

An aside comment - while I agree it would have been nice to have seen Maple involved in Numb3rs, I can tell you that despite its popularity, much of Caltech, i.e., the home of Numb3rs, is moving away from MATLAB. Python with Numpy, etc. is free.  This is leading us to a quandry. While we use Maple to introduce students to the idea of a modern problem solving environment, we eventually have them move to MATLAB because of its popularity. Now we wonder.

@rlopez Agreed - after this discussion, I am considering having the students import the Student Vector Calculus library.

My philosophy is to try to limit the number of Maple specific lines. I can pretty much cover 3 semesters  of introductory physics (mechanics, electromagnetism and quantum mechanics) using only two libraries - plots and Statistics. When I teach upper division courses, the number of routines increases. For quantum mechanics, students add routines  in LinearAlgebra and Physics. Physics was a big hit because of its ability to perform algebra using Dirac notation. 

@Carl Love I appreciate your explaination. After I wrote the question, I started to wonder how Maple distinguished differences between a vector, which matches the format of most textbooks, and a list, which works, but lacks the pedagogical advantage of a vector. 

I also want to thank folks for putting up with my inquiries. These questions come from the less discussed reference perspective; that is, from who isn't trying to solve complex mathematical problems, but rather one who is using Maple as a powerful mathematical tool for educating students in applied mathematics and physics.

@Preben Alsholm I think you identified the issue - fieldplot was written originally for lists, but clearly vectors with the vectorfield attributes as an acceptable option was added later. A vector that lacks the vectorfield attribute makes it fail. However, as the help on VectorCalculus states, some procedures in the library require a vectorfield, but accept vector-valued operators. However, from a pedagogical standpoint, how it gets implemented is a pain: defining the constant in a vector before defining the vector is okay, but it isn't in the reverse order.

Sigh. Thanks for the suggestions.

@Kitonum 

My apologies, I’m tired thus misstated the problem. Yes, I do know it has to be a vector field. That isn’t the issue.  The issue is the lack of evaluation of the constant.

restart;
z:=A*<x,y>;
A:=1.0;
plots:-fieldplot(z, x=0..1, 0..1);
--> Error, (in plots/fieldplot) unable to obtain field plot, additional unknown(s) found: {A}.

But the work around is define A first before defining z.

restart;
A:=1.0;
z:=A*<x,y>:
plots:-fieldplot(z, x=0..1, y=0..1);

or not assigning a value to A and using  eval() to create a temporary value for A.

I don’t understand why fieldplot won’t use z with the evaluated value for A. In fact it become a real pain if I want to create a field like:

z:=<A*x, B*y>:

where A and B are changed several times in the worksheet. 

@sand15 I agree that just looking at the examples of how employs quotes isn't enough to understand their defintion. I found the help page ...helpful:

https://www.maplesoft.com/support/help/Maple/view.aspx?path=quotes

@Mac Dude 

Thanks Mac Dude for your thoughts. A goal of my inquiry was to answer the question – am I missing something. It sounds like I may not.

I teach courses that range from introductory physics to upper division quantum mechanics. I absolutely agree with you that part of being familiar with a concept is to work with it, but not to spend gobs of hours plowing through mindless algebra or revisiting tables of integrals. Your approach to using Maple appears to be similar to mine, though you are more organized I given that you provide them with a spiral-bound book. I am often modifying last year’s worksheets the day before class based upon the abilities of the particular class. My students general like Maple, since it is readable, provides a record of calculations, and makes pretty plots and animation. Once they realize that have to be precise in wording and labeling (case-sensitive), I see them using it in other courses. And the beauty of introducing Maple early in the intro courses is that by the time the students are taking quantum mechanics, I can simply say: solve this problem, forcing them to write the worksheets from scratch.

With regards to my inquiry, I may be teaching some non-science majors courses and so canned applications are very useful. PhET at the University of Colorado is a prime goto website for the types of applications I envision writing.

It looks like we have similar takes on MapleCloud, though yours is based on actual experience. I appreciate your response.

@acer Thanks for your suggestion which does satisty the conditions. As an educator who has to explain this to students, I prefer if rules regarding the entry of parameters be consistant and should be consistant in the help. The help says: fieldstrength = log, which I believe should be entered as a string.It does not allow me to pass a function. For example, I can't say "fieldstrength" = "exp".  Hence I consider this outcome as a bug.

@Carl Love Worth a try, but the output is: log, ln
Which appears to tell me nothing.

Here is the code:

restart;
plots:-fieldplot(<x,y>, x=0..1, y=0..1, fieldstrength=log);  #which works
with(RealDomain):
plots:-fieldplot(<x,y>, x=0..1, y=0..1, fieldstrength=log); # which fails

@Carl Love 

Your suggestion is great for the display. Thanks. However, larger images on the display produces a plot fills an entire page upon printing.  On my QHD display, to print a plot (or create a PDF of a plot) that is less than half the size of the page, I have to resize the plot to nearly a postage stamp. Do you know of an option to resize the plot upon printing?

I like large monitors, so I'm happy with double 27 inch versions, though I would love to move to a single 34 inch monitor. 

However, what I can say that I don't recommend is a quad-high-density (QHD) monitor. I have such a screen on the laptop. Plots ave very tiny and too thin. If I expand the size of the plot, then a PDF of the worksheet makes the plot huge! In addition, if I want to see a print-preview, the image is far too small and is clipped. 

Sorry, nothing shown for the Maplet. :-(

Note, there is an error in the worksheet. Amplitude is the 1 / sqrt(Normalization_integral), i.e., A:=evalf(Normalization_integral)^(-1/2).