Napoleon in sunglasses
Q: A fantasy novel protagonist, who is fluent in French, has snapped back to modern times after carrying two polarized sunglasses on a trip to France in the early 1800s. The protagonist rushes into an art museum to see if the mischief during the trip worked. Sure enough, Napoleon on horseback is now wearing sunglasses. What French physicist, military officer and contemporary of Napoleon did the protagonist visit to help him scoop Edwin Land by over 100 years? What demonstration did the protagonist do with the sunglasses to show the physics at work? Answer in English. Googling is allowed.
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From the December issue: RUNAWAY PLANE
We asked you to review a fictional screenplay about a conventional airliner going supersonic. Your answers were reviewed by AIAA Fellow Jim Kuchar of MIT’s Lincoln Laboratory, who provided the idea for this question.
WINNER: Groundspeed is not airspeed. A sonic boom occurs when an object moves through the air faster than sound can. The ratio of the speed of an object through a fluid divided by the speed of sound in that fluid is known as the Mach number. Above Mach 1, pressure waves cannot move as fast as the object, build up like the bow wave of a fast-moving boat and reach the ground simultaneously as a “boom.” Mach number ignores any velocity the object and fluid share. Therefore, it does not matter what the tailwind is for our fictional airliner. Unless the pilot changes his throttle or altitude, his Mach number is constant and there is no danger of a sonic boom, despite the tailwind giving him a large increase in groundspeed. In 2019, a Boeing 787 Dreamliner broke the airline groundspeed record at 801 mph [1,289 kph]. This may seem supersonic, given the speed of sound in ambient air is 740 mph [1,191 kph]. However, that plane had entered a similarly record-breaking 230-mph jet stream. To set that record, it would have only had to fly at 571 mph inside the jet stream (around Mach 0.8, or 80% of the speed of sound), which is only 10 mph above its cruising speed. With a top airspeed of 587 mph, it would be impossible for a Dreamliner — or any other current airliner — to go supersonic in level flight.
Jeffrey J. Mach, AIAA senior member
Santa Clara, California
Jeffrey works for Sierra Lobo Inc. as a site manager at the Thermophysics Facilities Branch of NASA’s Ames Research Center.