Im taking mechanics of materials. One of the things they talk about is cutting out a small elemental cube of a rigid body, that has sides dx,dz,dz. Is it always true that dy,dx, and dz have the same infinitesimal size? I thought that they would not necessarily be the same size, which could give you a rectangle. The reason I thought this is say you have say a rectangular box, and cut it with a grid pattern, and you make the grid finer and finer. Then if its longer in the x direction than the y direction, a rectangular box, and I make my grid all squares based on the smallest dimenson, the y direction, then I can shrink all the squares more and more. It is clear that as my grid shrinks, I will approach dy much faster than I approach dx. I would expect to get to dy first, as y is the smaller direction, and dx much later, if its x>>y, since I cut it into cubes and made those cubes finer and finer.