Fluid Statics - net upward force

AI Thread Summary
The discussion focuses on calculating the net upward force on a cube submerged in a liquid, using the principles of fluid statics. The net force is derived from the pressure difference between the top and bottom faces of the cube, resulting in the equation Fn = ρlg. Participants express confusion over the variable h, questioning whether it differs from the initial depth h0 mentioned in the problem statement. Clarification is sought regarding the relationship between h, mass m, and the cube's dimensions, leading to the equation h = m/ρl². The consensus is that h should represent the depth at which the cube floats when released, rather than the initial depth h0.
Roodles01
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Homework Statement


Find magnitude of net upward force on a cube in terms of l, ρ (rho), g and h. then show that h = m/ρl2

A solid cube, mass m, side length l, is placed in a liquid of uniform density, ρ(rho), at a depth h0 below the surface of the liquid, which is open to the air.
The upper and lower faces of the cube are horizontal.

(this is from previous question https://www.physicsforums.com/threads/force-exerted-by-fluid.784711/)

Homework Equations


pressure, p = ρ g h
mass = ρ x volume of liquid displaced

The Attempt at a Solution


Net force, Fn = (pbottom x area of cube face) - (ptop x area of cube face)
p at top face = (pT) x ρliquid x h x g
p at bottom face = (pB) x ρliquid x (h+l) x g
so
Fn = (ρgh + ρlg - ρgh)
Fn = ρlg

Net force - fine.
Er! how do I show that h = m/ρl2
I know m = ρ x v . . .
 
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Roodles01 said:
Er! how do I show that h = m/ρl2
I can only make sense of the question if h is different from h0. (Your working all has h, but the question used h0.)
A reasonable meaning for h would be that the block is released and floats level with depth h immersed.
 
haruspex said:
I can only make sense of the question if h is different from h0. (Your working all has h, but the question used h0.)
A reasonable meaning for h would be that the block is released and floats level with depth h immersed.

I have looked at the question and unscrewed the bits of paper I attempted my answers on and the h0 seems to be a complete typo, it all says just plain h. I will try to work through with that in mind, as you say.
 
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