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Force and weight

  1. Jan 12, 2004 #1

    Okay, there are a bunch of flies 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.

    I can't organize my thoughts! I can't explain this.

    Any help will be greatly appreciated!
  2. jcsd
  3. Jan 12, 2004 #2


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    The flies are suspended in air, the air forces down on the jar. That's assuming the flies are not performing some collective motion. For example, if they all suddenly dive to the bottom, the jar will be momentarily lighter, regaining normal weight when they land.
  4. Jan 12, 2004 #3
    Yeah, that is what I was thinking...it is just driving me crazy because I can't collect my thoughts as to the "scientific" reason this is so. Hmm...thanks though!
  5. Jan 13, 2004 #4
    Well, for a scientific reason, think about this. The mass of the jar (which includes the flies inside of it) does not change depending on the location of the flies. Unless they perform some act like krab said.
  6. Jan 15, 2004 #5
    Agreed. The answer, without special circumstances, is C.
  7. Jan 20, 2004 #6
    First, if you consider the MASS "jar+flies", that is definetly a constant whatever the flies are doing: sitting or flying.

    Now, if you consider the weight of the "jar+flies", that means you are looking at force that is exerced on the scale. There is there tree contribution to that force: from the jar (constant force), from the air in the jar and from the flies in the jar. Now do not forget that the flies rely on the air to be able to fly. Therefore, whether they sit on the bottom or they fly should not make any difference. In the air, the wings of the a fly exert on average a force on the air which is transmitted to the bottom of the jar and therefore to the scale. If there was no air (vaccum), the fly could simply not fly and would always rest at the bottom.

    An easier picture would be to consider the jar filled with water whith a fish swiming in the jar. It would not matter if the fish float at the surface, sit a the bottom or swim in the middle: the fish weight applies a force on the water which is transmitted to the bottom of the jar.

    Note: you may find that the force is not always constant in time and that the force is fluctuating during the measurement. That is because when the fish/flies move around they exert additional forces necessary to accelerate/decelerate their motions. However, these fluctuation should average to zero and the average weight in time should be "jar+air/water+flies/fish". Think that it is the same when you try to measure your own weight on a scale but you do not stay still...
  8. Jan 20, 2004 #7


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    If the flies are all accelerating upwards, the jar will register the highest value.
  9. Jan 21, 2004 #8
    And if they are accelerating downwords - stooping ?
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