GeekDad Puzzle of the Week: This is a Non-No-Fly Zone — Solution

Reading Time: 3 minutes

Photo: Prelinger Archives via Wikipedia; Public Domain

Puzzle

Mr. Lion, the eccentric billionaire who recently bought and modified an island in the Bermuda Triangle has selected a site for his mansion and is having new roads built. He failed to conduct a thorough survey and has only recently discovered a large canyon cutting directly across the path of one of his roads. A temporary suspension bridge has been constructed to allow for the transfer of equipment and supplies to and from the work site where a more permanent bridge is being built. The suspension bridge is strong enough to hold a cargo truck with a mass of exactly 5,000 kg (gross), but no more (this is the engineering constraint, and there is no margin). The bridge is 10 km long. Because many of the vehicles used to transport materials across the bridge are arriving at very near to that capacity in mass, a scale was built at the entrance to the bridge and the vehicle mass is measured just before being allowed onto the bridge.

The first truck of the day arrives and weighs in at exactly 4,999.5 kg. The driver and navigator are waved through (the guard hasn’t had his third cup of coffee yet). Acrophobia causes the driver to go slowly and insist on running the air conditioner with the windows up. At the center of the bridge, the driver is a wreck. The navigator offers to drive for a while, and the driver agrees. They stop and shut off the truck and begin switching sides (not leaving the truck to do so). Before they can finish (they’re in a tangle of limbs), they notice a Yellow-Crown Night Heron descending on the truck, in an apparent attempt to land on it. The navigator is an avid birdwatcher and knows that the typical adult has a mass of about 625 grams and this bird appears to be an adult in good health. The navigator realizes that this bird plus the mass of the truck exceeds the strength of the bridge. He tries the horn to startle the bird out of landing, but it’s not working. He yells, and makes as much noise as possible, but the bird does not seem to hear. He watches desperately as the bird alights.

What should they do to stop the bridge from collapsing in the fraction of a second they have left as the bird lands on the truck?

Solution

The simplest answer is that they need do nothing. The mass of fuel this big truck consumed during its slow, air conditioned, 5 km trip is much greater than the mass of the bird, so they’re safe. I received several answers about throwing shoes or other items out of the truck, but remember that the windows are up and they have no time to roll them down before the bird lands. Similarly for the idea of dumping the cold air from the cab: there’s a slight increase in density of the cabin air caused by air conditioning. The cold air in the cabin is about 4% more dense than the warm air outside, assuming 90 degrees F outside and 70 degrees F inside, but they just wouldn’t have had time to roll down the windows or open the doors before the bird landed.

Anyway, thanks for playing! Please come back on Monday for our next puzzle. And go get \$10 off your \$40 or more purchase at ThinkGeek by using the code: GEEKPUZZLER

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