Originally Posted by

**bobbyk** Here's a rather interesting problem in fluid mechanics.

A hollow weightless infinitely thin spherical shell (yes, this IS an ideal problem)

is evacuated and tied with a string to the earth's surface, where the gravitational acceleration is g. Of course, there will be an upward tension in the string of m*g, where m is the mass of the displaced air.

The string is then cut. Assuming the air is an ideal gas, what is the INITIAL

upward acceleration of the sphere? Some would say g, but this is wrong,

since it doesn't account for the acceleration of the air adjacent to the

sphere.