Slide 8 of 12
Notes:
We are simplifying things a little here by assuming that there are only two forces of support. The results, though, are equally true for a continuous set of support forces.
The reason both support forces must push up, not down, is that materials of which buildings are made (stone and mortar for the tower of Pisa) can sustain compressive stress, but pull apart under the slightest tension. The same is true for the forces between earth and building foundation -- it’s usually easier to lift something off the Earth’s surface than it is to push it down further.
My grandchildren sometimes try to see how high they can make a tower of toy blocks. As the block tower gets higher and higher, it’s harder and harder to keep the center of gravity of the tower of blocks directly above the bottom block’s “footprint,” and that’s what always makes the tower fall over, in the end.
