A Normal Sidewalk?

I've seen this pattern in many sidewalks. A crack forms at the seam of two pieces of concrete. Over time and use the crack crumbles away portions of the sidewalk in a bell-shaped pattern. What process or differential equation might define the resulting bell-shaped pattern of wear?

The Density of Space Trash

Here is an image showing the density of space debris in low earth orbit. Notice that this density has a very long, and likely heavy, tail that is not shown. Geo-synchronous satellites orbit at 36,000km well off the upper range of this graph (2,000km). There would be a large bump in the density at 36,000km. From The Washington Post.

A Pearl: a Balanced Median Necklace

The September issue of The College Mathematics Journal has this balance point view of the median by Mark Lynch of Millsaps College.