Fluid Statics - net upward force

In summary, the question is asking for the magnitude of the net upward force on a cube with side length l and mass m, placed in a liquid with density ρ at a depth h below the surface. Using the equations for pressure and mass, the net force can be calculated as ρlg, and it needs to be shown that h = m/ρl^2. This can be done by considering the block floating level with a depth h immersed, assuming h is different from h0, as mentioned in the conversation.
  • #1
Roodles01
128
0

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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  • #2
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.
 
  • #3
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.
 

Related to Fluid Statics - net upward force

1. What is fluid statics?

Fluid statics is the study of fluids at rest, specifically looking at the forces acting on a fluid and the resulting equilibrium.

2. What is net upward force in fluid statics?

The net upward force in fluid statics is the sum of all the forces acting on a fluid in an upward direction. This includes forces like pressure, buoyancy, and surface tension.

3. How is net upward force calculated?

Net upward force can be calculated using the equation F = ρghA, where ρ is the density of the fluid, g is the acceleration due to gravity, h is the height of the fluid column, and A is the cross-sectional area of the column.

4. What causes net upward force?

Net upward force is caused by the difference in pressure between the top and bottom of a fluid column. This pressure difference is due to the weight of the fluid above and is what keeps the fluid column in equilibrium.

5. How does net upward force affect objects in a fluid?

Net upward force can affect objects in a fluid by either pushing them up, if the object is less dense than the fluid, or pushing them down, if the object is more dense. This is the principle behind buoyancy, where objects float or sink based on this net upward force.

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