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Why does the Upward Hydrostatic Force of a submerged object = Fg of the water column above it + Fg of the volume of displaced water?
I would imagine the Upward Hydrostatic Force to be equal to Fg of the water column above it + Fg of the object. This is because my brain tells me that the upward hydrostatic force is just like normal force:
All that is pressing on the bottom of the submerged object is the Fn of the water above it and the Fn of the object itself.
Specifically, why is it NOT the case that Upward Hydrostatic Force equals Fg of the water column above it + Fg of the object?
Please don't just say "pressure increases with depth", that's no help. I need an intuitive understanding here. What is propelling the object upwards on a molecular level?Thank you.
1 Answer
- Andrew SmithLv 78 months ago
I answered one of these and was given a thumbs down for my trouble. I hope YOU weren't the one. On a molecular level the water molecules are bouncing around in every direction. They are bumped down by those above them and up by those below them. These two forces must be equal or the water must move up or down which is impossible.
Now put an object there. So there are no water molecules pushing down. There are water molecules below pushing up. With exactly the same force as before. So what stops the water from going up? The object. Now unless the object pushes down with the SAME force that the water below is pushing up then the water immediately below the object must accelerate upwards taking the object with it. To keep the object stationary the total forces of the object must balance the upward molecular forces.
ie the force of the water above the object plus the force of gravity on the object plus any downwards ( or subtract any upward force ) applied to the object externally must always equal the molecular forces pushing upwards on the object or the object will accelerate ( float up or sink ).
I could go into more detail but I don't think that is relevant to the specific question.
