You also need to know what the Neutral Axis of the beam is. The neutral axis
is the 'center' around which the entire beam flexes. Draw a 4 unit wide by 2
unit tall beam.
The applet recalculates the center of the beam every time you add or remove a square, and shows the center as a cross. The applet also shows as heavy black bars at the edges the overall dimensions of the drawn elements.
You will notice that there is a small box in the upper right corner of the
applet that displays various different pieces of data. Among them are the area
in square units, and the center point as an (x,y) coordinate. There is also an
indicator of how far the top and bottom are from the center point (Y-bar Top
and Y-bar Bot, respectively).
Keeping the 4w by 2h beam on screen, add another unit connected to the bottom
of the beam similar to this one: 
.Did you notice
anything change in
the data box, other than the
center? There are two other lines in the box we didn't define yet. Can you
guess their purpose in life?