A question you won't see on the World's Smartest Design Engineer game

It can be exasperating to come up with questions for our World's Smartest Design Engineer game. Not only must we be sure of the correct answer, but we must also be sure the answers listed as incorrect are, in fact, incorrect.

An example illustrates the point. One of our editors recently submitted the following question:

"A large man and a petite woman would like to parachute at equal terminal velocities. What does the man need to do?"

A. Use a bigger parachute

B. Jump softly

C. Pull up on the supporting strands to decrease downward force

D. Jump out of the airplane first

The editor insisted that A, use a bigger parachute, was the right answer. But not so fast. We ran this question by an experienced parachutist who has well over 100 jumps to his credit. He claimed "use a bigger parachute" is absolutely the wrong answer. It would make the man more dense. If anything, the woman would be the one who would have to have a bigger parachute. But the real way parachutists typically address this problem, he says, is with special jump suits.

Well, back to the physics book which inspired the original question. It turns out that the difficulty was in the interpretation of 'terminal velocity.' Our parachutist friend naturally assumed the question pertained to parachutists in free fall who were trying to maintain identical altitudes on the way down. The physics book used the term to mean something quite different. To its author, a parachutist's 'terminal speed' was their speed with the chute open:

"Consider a man and a woman parachuting together from the same altitude. Suppose the man is twice as heavy as the woman and that their same-sized parachutes are initially opened. Having parachutes of the same size means that, at equal speeds, the air resistance is the same on both of them. Who reaches the ground first -- the heavier man or the lighter woman? The answer is that the person who falls fastest gets to the ground first -- that is, the person with the greatest terminal speed. At first we might think that, because the parachutes are the same, the terminal speeds for each would be the same and, therefore, that both would reach the ground at the same time. This doesn't happen, however, because air drag depends on speed. Greater speed means greater force of air impact. The woman will reach her terminal speed when the air drag against here parachute equals her weight. When this occurs, the air drag against the parachute of the man will not yet equal his weight. He must fall faster than she does for the air drag to match his greater weight. Terminal velocity is greater for the heavier person, with the result that the heavier person reaches the ground first."

So, after careful consideration and too much time, we decided there was no way to word this question so it wouldn't be at least slightly ambiguous and still remain within the confines of the 250-character limit imposed on questions by the game software.

Of course, if you want to see questions that made it through our screening process, you can play the game: www.smartestdesignengineer.com

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Lee Teschler

Leland serves as Editor-in-Chief of Machine Design. He has 34 years of Service and holds a B.S. Engineering from the University of Michigan, a B.S. Electrical Engineering from the University of...
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