Many thanks Pagan, your answer was very useful. Thanks Epostma too for answering.
Yes this was the resulting volume of revolution I was looking for.But I'd rather prefer a more direct way to find it.
Definetely Maple 11 Classic has a bug here.
I looked at the maple help section.
About VolumeOfRevolution it says:
"The VolumeOfRevolution(f(x), x=a..b) command returns the volume of revolution of the expression f(x) from a to b."
Which is not true.
If you plot
VolumeOfRevolution(2*x^2,x=0..2,axis=vertical,'output'='plot), the program instead returns the volume of revolution of 2*x^2 from y=0..8.
Next, the help section says:
"The VolumeOfRevolution(f(x), g(x), x=a..b) command returns the volume of revolution between the expressions f(x) and g(x) from a to b."
Which is not true either.
If you plot
VolumeOfRevolution(2*x^2,0,x=0..2,axis=vertical,'output'='plot), does not return the solid of revolution,between 2*x^2 and y=0 between x=0..2.
So I found this other command:
with(Student[Calculus1]):
> VolumeOfRevolutionTutor(2*x^2,x=0..2);
I get the same wrong plots than before. Instead if I choose the shells option and increase the "number of partitions" i get the right plot, in a straight forward way.
Now if in the tutor I try to plot f(x)=2*x^2, x=0..2, axis=vertical distancefromaxis=8, I get the wrong solid again.
Choosing the disks option doesn't help much this time.
If I add g(x)=0 I get a "better" aproximation.
Maple 11 classic is not working properly when it has to plot this volumes of revolution.
Also in the next example maple is not even able to calculate rightly the volume.
I want to plot and calculate the volume of rotating around x=6 the area between the functions y=6-x, y=4,from x=0 to x=6.
I open the VolumeOfRevolution tutor. I write f(x)= 6-x, g(x)=4 a=0 b=6. "Vertical" and "Distance from..."=6
The graph is wrong and the volume given is wrong too. The tutor result for the volume is 128*Pi/3=134.04.
If you calculate it manually the right result is 368Pi/3=385.36.(Pi*Integral(6^2-(6-(6-y))^2)*dy
I would be nice if Maple could arrange this.
I hope this help people not to get mess up with their plots when they first draw them with Maple 11 classic.

"I think those two are the same - the same curve is described by y = sqrt(x) for x=0..4 and x = y^2 for y=0..2. How the surface of revolution looks does not depend on the way in which you write it."
yes, the function is the same. But the areas are diferent.
Though the function is the same, the area limited by x=y^2, x=0 and y=2 is different than the area limited by x=sqrt(x),y=0 and x=4.
If you rotate the first area around the y axis you get the solid plot by:
VolumeOfRevolution(sqrt(x),x=0..4,axis='vertical','distancefromaxis'=0,'output'='plot'); (Which is not the right plot)
I want to get to plot the solid of revolution that you get when you rotate the second area (x=sqrt(x),y=0 and x=4.)
around the y axis.
About this plot,
"VolumeOfRevolution(y^2, y=0..2, axis='vertical', output='plot');"
It does not make any difference the name of the variable.
That's the same than
VolumeOfRevolution(x^2, x=0..2, axis='vertical', output='plot');"
or
VolumeOfRevolution(m^2, m=0..2, axis='vertical', output='plot');
But it does not plot x=y^2.
How can I do it?