Problem of fluid -- Blocks, pulley, two fluids

[Queren Suriano]

Homework Statement

dynamic viscosity 1 = 1125 2 kgf-s / m2 , dynamic viscosity 2 = 62.5 2 kgf-s / m2 , Areas: A1 = 20 cm2 , A2 = 50 cm2
W1 = 15 Kgf, W2 = 40 Kgf.
The block (2) is displaced downwards with a
constant speed of 80 mm / sec, whereas
linear velocity distribution determined:
1) The weight of the block (3).
2) The speed of the block (1).
Neglect friction in the pulley

Homework Equations

the angle is 30º
(tangencial force) / area = (dynamic viscosity) (velocity/space betwteen the two surfaces)

sum forces=0 (Equilibrium)

The Attempt at a Solution

I did the problem of two ways, considering each block and considering the system, but I don't get the same answer.

What am I doing wrong??

 

[Chestermiller]

Your force balance on block 2 is incorrect, and your force balance on the overall system is incorrect. Your calculated value of F2 is correct. The force balance for the overall system should be the sum of the force balances on the individual blocks.

Chet

 

[Queren Suriano]

I don't understand why the force balance on block 2 is incorrect?? Because in the block 2 acts two tangetial forces due the fluid, and the component horizontal of the weight

 

[Chestermiller]

I don't understand why the force balance on block 2 is incorrect?? Because in the block 2 acts two tangetial forces due the fluid, and the component horizontal of the weight

It should include only the component of the weight of block 2, not both blocks.

Chet

 

[Queren Suriano]

It should include only the component of the weight of block 2, not both blocks.

Chet

But the block 2 support the block 1, if I only consider the weight of block 2, it's just like the block 2 was alone, but it doesnt

 

[Chestermiller]

But the block 2 support the block 1, if I only consider the weight of block 2, it's just like the block 2 was alone, but it doesnt

The weight of block 1 exerts a force on block 2 in the normal direction, but block 1 is not exerting a force on block 2 in the tangential direction. The fluid in the gap between block 1 and block 2 is exerting a force on block 2 in the tangential direction, but you've already included that in your force balance.

Chet