Jeopardy! category: MAY THE FORCE BE WITH YOU (04-11-2015)
$800 clue: Pushing through the air at more than 1,300 miles per hour, the Concorde expanded 8 inches in length during flight because of heat generated by this contact force
The gist: The skin type, specifically. Skin friction is one of the five basic types of friction, along with dry friction, fluid friction, lubricated friction, and internal friction. All friction is the resistant force that acts when two objects in contact move in relative motion to one another, converting kinetic energy into thermal energy, like rubbing two sticks to create fire. Skin friction is the force that acts upon the “skin” of a body (in this case, the airplane’s chassis) by a fluid that the body is going through (in this case, the air… hopefully). As one of the two components of drag, skin friction can be defined by the drag equation, meaning it increases as surface area exposed to the fluid increases. This is (mostly) why ultra-aerodynamic airplanes like the Concorde had such sleek looks to them, and why a semi truck will experience more skin friction (and correspondingly more drag) than a low-to-the-ground sports car.
The clue: “Friction” is clearly enough for this one; if the writers wanted “skin friction,” they’d have to have written the clue much more specifically than it is. Otherwise, it’s a pretty basic physics clue, with a nice aeronautic backdrop as eye candy.
In Jeopardy!: Friction is a pretty common topic in Jeopardy!, with about 40 clues in the J!Archive, but it appears in a wide variety of clues. Naturally, the most common theme is forces that resist motion, and others include converting energy to heat and lubricants. Beyond that, no real patterns.