Question

Robert is driving his convertible at the 65-mph speed limit. The car's soft, elastic ragtop roof is in the "up," or closed position, and the car's windows are closed. What will an observer most likely see happening to Robert's ragtop roof as the observer watches Robert's car move along the highway?


A. The roof will bow inward toward the interior of the car.

B. The roof will appear no differently than it would if the car were at rest.

C. The roof will bow outward, away from the interior of the car.

D. The roof will bow inward, but only when Robert is driving uphill.

I'm having a hard time deciding between B and C. Copy and pasted answers will not be chosen. Please explain your answer. Thanks!

Answer 1

It should lift (bow outward), but in only a very small way, due to Bernoulli's principle whereby the air moving faster above an object relative to beneath the object causes a low pressure zone (in this case the "above" is the roof, and "below" is beneath the car). In the case of Bob's car, this low pressure zone will be exploited by a minute change in the interior pressure that pushes the ragtop out and away. However, as even a cheap convertible's ragtop is quite rigid, I'd be very surprised if anyone would actually notice it. I know my Jeep doesn't (of course, Jeeps aren't exactly speedsters, either).

So... I think that you need to choose between (3), the minute bulge caused by Bernoulii's principle, or (2) it will appear no different to an observer, because that bulge is so incredibly small