Four compasses are placed around a wire, as shown.

A long wire that is vertical and standing on a steel plate. There are 4 compasses surrounding it and all of the needles are pointing in the same direction, east.

What is the best conclusion that can be drawn?

Engines for propeller-driven aircraft are limited in their maximum rotational speed by the fact that the tip speed of the propeller must not approach the speed of sound in air (mach i). taking 6 ft as a typical diameter for a propeller of a light airplane and 1100 fils as the speed of sound, find the upper limit on the rpm (revolutions per minute) of the propeller shaft.