A scientist weighs a jar of sleeping flies. Then, he shakes the jar to wake them up. While flies are flying in a closed jar, the scientist weighs the jar again. Does the jar weigh less when the flies are flying?
A fly is flying by pushing air downward. The fly applies a downward force on the air equal in magnitude to its weight mg. According to Newton’s 3rd Law, the air exerts equal and opposite force on the fly. Therefore, the fly stays in the air.
Now, the force is exerted on the air in the closed jar transferred to the walls of the jar. And finally, the force mg, the weight of the fly, transferred to the scales.
In conclusion, the jar weighs the same when the flies are flying.
Image By Blake Facey