Calaculate Mass of Cube submerged in oil and water

[TFM]

[SOLVED] Calaculate Mass of Cube submerged in oil and water

Homework Statement

A cube is submerged in water and oil the oil is on top, and forms a layer 10cm thick. the water is underneath and forms another layer 10cm thick. the cube has sides of 10.2cm, and has 2.3cm below the oil water line. the density of the oil is 790kg. I have worked out already the force on the top of the cube is 163 Pa, and on the bottom 1000 Pa. How do you work out the mass of the cube?

Homework Equations

Pressure = Force/Area

Archimedes: The upward force is equal to the weight of fluid displaced by the block (?)

The Attempt at a Solution

Any help would be appreciated

TFM

 

[Shooting Star]

Use Archimedes' separately for the two parts submerged in the two liquids and add the buoyant forces to balance the weight. Hence, find mass. No need to find pressure.

 

[TFM]

Am I doing simetghing wrong? I am doing:

F = p*A
A = 0.102^2 = 0.0104

P top = 163
Pressure Bottom = 1000

F top:1.696
F bottom = 10.404
Sum F = 12.10

using F = mg,

m = 1.23kg

But it is wrong?

TFM

 

[Shooting Star]

Am I doing simetghing wrong? I am doing:

P top = 163
Pressure Bottom = 1000

TFM

Show how you got these two.

 

[TFM]

Therehave been stated as being right, because I am using mastering physics, and have previously calculated them and got them marked correct.

I used P = denstiy*g*height,
Density Oil = 790 (stated in question)
height I calculated as being 2.1cm (0.021m)

Works out as 163 Pa (Did same thing for Water at the bottom, but used the total pressure = pressure caused by the oil on top + the pressure from the water layer from the join to the bottom of the cube.)

The Pressures are gauge pressures, by the way, if that helps?

TFM

 

[Shooting Star]

Am I doing simetghing wrong? I am doing:


F top:1.696
F bottom = 10.404
Sum F = 12.10

using F = mg,

m = 1.23kg

But it is wrong?

TFM

Why have you added the forces? Net force acting upward should be F_bottom - F_top.

I still recommend that you learn how to solve using the method I had told you right in the beginning. If the body given is not a cube or parallelepiped, then you won't be able to do it using your way.

 

[TFM]

Thanks. Finally got the right answer (0.889kg)

TFM