Will a jar with a fly in it be heavier than a jar without a fly?
Sometimes physics teachers like to “troll” students and ask questions like “will the weight of a jar with a fly inside change if the fly flies,” informs Ukr.Media.
To answer this question correctly, it is enough to have an idea of the general course of mechanics. You can, of course, understand the issue by conducting an experiment. But it is much more interesting to try to predict the behavior of the system, relying on theoretical knowledge.
The key to the solution is simple. Let's figure out how a fly flies? Imagine that there is no air in the jar at all. Will it be able to fly at all? Of course not! After all, the entire process of flight is based on the fact that the fly “rests” its wings on the air and pushes off from it.
To stay in the air, the fly has to push the air downward with a force equal to its weight, and this force is transmitted to the can through air pressure.
It turns out that the weight of the fly is always supported by the can, directly or indirectly. There is no physical difference between the situation where the fly is simply sitting on the bottom of a hermetically sealed can and the situation where the fly is flying inside the same can.
But it's not for nothing that we highlighted the phrase “hermetically sealed.”
The weight of a can of fly can still vary and here's why.
The formulated answer assumes that the can is perfectly airtight and does not deform, and that the air inside the can is at the same temperature and pressure as the outside air. If any of these conditions are not met, then the weight of the can may change slightly during the fly's flight.
For example, if the can is not sealed tightly, some of the air may escape when the fly pushes it down with its wings, which will ultimately reduce the weight of the can. If the can is not rigid, it may expand or contract slightly as the fly changes position, changing the volume and density of the air inside the can. Or, if the air inside the can is hotter or colder than the air outside, a different buoyant force (different from the external force) may act on the fly, affecting the weight of the can.
These effects are very small and difficult to measure, but they are not zero.
One such experiment was conducted by Dr. John Norton of Carnegie Mellon University in 2023. He used precision scales to measure the weight of a sealed jar with a fruit fly inside. It turned out that the weight of the jar fluctuated by about 0.0001 grams during the flight of the fly, depending on how fast and high it flew. It also turned out that these fluctuations were caused by changes in temperature and pressure inside the jar, not changes in gravity or inertia.
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