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Will the weight of jar changed?

  1. Jul 24, 2010 #1


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    I read the following problem in a book

    "A bunch of flies are in a capped jar. you place the jar on a scale. The scale will register the most weight when the flies are
    a) sitting on the bottom of the jar
    b) flying around inside the jar
    c) ... weight of the jar is the same in both cases"

    The answer from the book about this problem is c). But I don't understand if the flies are sitting on the bottom, I think we should add the weights of flies even it is so tiny (if we consider the scale is sensitive enough for no matter how small the weight is?). But why the answer will be c)?
  2. jcsd
  3. Jul 24, 2010 #2


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    Newton's 3rd Law of motion. Flies fly by pushing down air. Air pushes at the bottom of the jar.

    Mythbusters actually tried this one with birds in a truck.
  4. Jul 24, 2010 #3


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    You do include the weight of the flies.

    You include the weight of the flies both when they are at rest on the floor of the jar and when flying around in the jar.
  5. Jul 24, 2010 #4


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    When the flies are flying, then the pressure differential within the jar is increases so that the pressure at the top is a bit less and the pressure at the bottom is a bit more, and the total force created by this pressure differential will exactly equal the weight of the air and the weight of the flies.
  6. Jul 25, 2010 #5
    Actually the weight can decrease when the flies are flying up.

    When a fly is flying then suddenly accelerates upwards, the jar will "weigh more". Also when the fly bangs its head on the jar cap, it will "weigh less".

    Also there is a delay in the pressure wave created by the wings until it strikes the bottom of the jar, but the speed of sound is large compared to the jar size.
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