On 10/28/2016 11:52 AM, Dan Asimov wrote:
Some physics questions I don't know the answer to:
1) An old Car Talk puzzle of the week is about a truck carrying a load of chickens.
As it approaches a mandatory truck scale, the driver realizes that its weight is just over the legal limit. To lighten the truck, he or she makes a loud noise, frightening the chickens so that they fly into the air just in time for the truck to be weighed. The chickens can only fly by pushing air down with sufficient momentum flux to equal their weight. So it couldn't have the desired effect in a closed truck. If would even be very hard for all the chickens to fly at once since they would create a circulation flow inside the truck that would work against them. In an open truck a small part of the flying chicken's pressure 'footprint' might fall outside the truck bed
Question: Does this succeed in lightening the load?
1a) Also, does it matter whether the truck is airtight or open? -----
2) In a related question, people on a sailboat realize that it needs to get back to shore in a hurry, so they attach a powerful fan to a powerful battery and aim it at the sail. Can this help propel the boat?
Not if they put the sail out for a reach and blow into it. But if they trim the sail as for a tack and direct the fan back over the outboard surface of the sail they would get some propulsion. But simpler is just furl the sail and point the fan astern.
2a) Likewise, suppose this picture is scaled down to become a small toy boat. Does it matter if all takes place in an airtight enclosed case?
If would make it less effective as the circulation of the air tends to be a counterflow. But here's a more interesting, and real world, question. If you build a landsailer (c.f. nalsa.org) but instead of a mast and sail you provide it with a big rear facing, variable pitch propeller (like an air boat) and you gear the propeller to the rear axle so it turns with the wheels, can you use it to sail directly into the wind? Brent Meeker