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Question
#24075. Bugsy
asks:
Galileo did not believe in the existence of atmosperic pressure. To support his view he would give the following reasoning. Imagine a container with liquid. Within the liquid consider a certain volume. The liquid inside this volume is acted upon by two oppositely directed forces: weight and buoyant force. According to Archimedes' Principle the two forces are equal. This is why the liquid is in {equilibrium;} it neither sinks nor floats upward. We might say, for example, that water immersed in water is weightless. How then can something weightless exert pressure on the underlying layers! Similarly air in air, being weightless, cannot exert pressure on the lower layers, and ultimately the surface of the earth. What is the flaw in his reasoning?
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Gnomon
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The flaw is saying that the water is weightless. Just because something is not moving downwards does not mean it is weightless. I'm sitting in my chair and am not moving downwards, but I am not weightless.
Nov 08 02, 8:48 AM
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Socrates
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According to Newton's third law forces always occur in pairs. Therefore, if the rest of the liquid exerts an upward buoyant force on the seperated volume, the latter must exert a downward force, equal to its weight, on the rest of the liquid. Thus pressure is exerted on the underlying layers. A similar reasoning can, of course, be applied to air.
Nov 08 02, 9:05 AM
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