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# Mensa Book

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Hello,My cousin recently purchased a Mensa book from a charity shop (thrift shop) which was published by British Mensa Limited in 1996. The book is called MENSA presents Logic puzzles.I would like peoples opinions on this question and answer:Question.Carrier PigeonsA driver approaches a bridge. He notices that the maximum weight allowed is 20 tons and knows that his empty pantechnicon (removal van ) weighs 20 tons. However, he has a cargo of 200 pigeons which weigh 1lb each. As the pigeons are asleep on perches he stops the vehicle, bangs on the side to waken the birds who start flying around, then safely drives over the bridge.Is he correct?Answer.No. The pigeons remain at 200lbs even whilst flying. Those flying up would reduce the weight, but those flying down would increase the weight, so balancing the total weight.end of answer.I don't understand this answer. For example, if it was a very tall vehicle and all the pigeons were flying upwards at the same time then does that mean the van weighs less than if they were all flying down at the same time? Surely the answer should have been Yes, the driver is correct. Or am I missing something?

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Each bird will have a mass of 0.454kg, and therefore a weight of 4.45N. If they are flying level, their lift will equal this force. If they are going downwards the lift will decrease, and if they go up the lift must increase. The resultant force exerted by each bird will vary depending on where it is moving. However, if they are not in contact with the van then they cannot impact its weight. For example, get a set of scales and put a mug or jar on them and record the mass. Now put your finger in the jar, but don't touch the sides or bottom. The scales will not change their reading. If you have really accurate scales (I tried this in a science lesson) then try blowing into the jar on the scales. You will see that the reading will change (you need very accurate scales - I used a 2dp accurate digital balance) due to the increased pressure.

So, the birds taking off would reduce the reading from the weighbridge. The air pressure created by the flapping wings would then cause an increase in the reading. Personally, I don't think it would make that much difference but I am interested to hear other theories. Also, if the birds took off and the guy drove onto the weighbridge, the birds would all hit the back of the van and probably get knocked out. Plus the person operating the weighbridge should really wait for the reading to settle down before taking it, which is unlikely to happen with a load of birds flapping their wings in the back.

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