Thread: find the moment of inertia

heres a very messy drawing for an idea of what were looking at: http://img89.imageshack.us/img89/1764/untitledyi0.png

area moment of inertia Ix0 of a rectangle about the axis x0 that passes through the centroid is: Ix = 1/12bh^3

the moment of inertia about an axis x that is parallel to the x0 is given by: Ix = Ix0 + Ad^2x

where A is the area of the rectangle, and dx is the distance between the two axes.

the inputs are width w, height h, and thickness t.

w = 200mm h = 300 mm, and t = 22mm

find the moment of inertia:

i know i need to use:

Ix0 = 1/12bh^3

Ix = Ix0 + Ad^2x


i've tried to put this together, but i need some help.


my questions are..

is t the what plugs into b in this problem?

also, is area of the rectangle just Base * height, so 200 * 300?

not sure how to find the distance between the two axis.

please help.

The distance between the axis is the vertical (or horizontal) distance between the centroid of the figure to the origin.
Now, the sign over here does not matter because anyway you are squaring.

But in general when it is important (like in product or inertia) you look if it is to right or left to determine the sign. Similarly when you do this with up and down.