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On an educational TV channel, a science program is explaining the properties of air. When the discussion moved on to the weight of air, a cartoon was shown on the screen which is shown here at the right. "When the balloon is filled with air, the scale indicates the balloon is heavier and so we can see that air also has weight." said the voice commentator. Do you agree with the explanation? Do you agree that the weight of the balloon has increased? |  |
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Now let's look at another example about similiar demonstration. A jam jar with a tight lid and with the internal air removed is weighted on a scale. Then the lid is opened and then closed so that the inside is now filled with air. Then we weight the jar again. The scale shows the weight of the jar has increased. So does this shows, clearly and without any doubts that air has weight? |  |
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Your trouble begins when someone asks you to open the lid and placed it on top of the jar. Now what change would we see on the scale? With no change in the reading of the scale, you can argue that only those part of the air insides the jar is being weighted but this time you won't believe it yourself either. Well, the answer to all these lies simply with the Archimedes' Principle which state that the upthrust on a body is equal to the weight of the fluid displaced by the body. In the first case, the expanded ballon displaced the surrounding air and so an increased upthrust resulted. The magnitude of the increase being equal to the weight of the air it displaced. If we ignore the thin thickness of the balloon surface, this is exactly the amount of air we put inside the balloon in the first place. The upthrust force is equal to the weight of the air inside the balloon so they cancelled each other. There will NOT be an increase in the weight of the balloon. Hard to believe? Why not try filling up an plastic bag all under water and check whether you can actually feel that the bag is now heavier, while your hand and the whole bag are still under water. |  |