A very large plate is placed equidistant between two vertical walls. The 10-mm spacing between the plate and each wall is filled with a liquid of absolute viscosity 1.92 x 10^-3 Pa.s. Determine the force per unit plate area required to move the plate upward at a speed of 35 mm/s. Assume linear variation of velocity between the plate and the walls.

Respuesta :

Answer:

Force per unit plate area is 0.1344 [tex]N/m^{2}[/tex]

Solution:

As per the question:

The spacing between each wall and the plate, d = 10 mm = 0.01 m

Absolute viscosity of the liquid, [tex]\mu =1.92\times 10^{- 3} Pa-s[/tex]

Speed, v = 35 mm/s = 0.035 m/s

Now,

Suppose the drag force that exist between each wall and plate is F and F' respectively:

Net Drag Force = F' + F''

[tex]F = \tau A[/tex]

where

[tex]\tau[/tex] = shear stress

A = Cross - sectional Area

Therefore,

Net Drag Force, F = [tex](\tau ' +\tau '')A[/tex]

[tex]\frac{F}{A} = \tau ' +\tau ''[/tex]

Also

F = [tex]\frac{\mu v}{d}[/tex]

where

[tex]\mu[/tex] = dynamic coefficient of viscosity

Pressure, P = [tex]\frac{F}{A}[/tex]

Therefore,

[tex]\frac{F}{A} = \frac{\mu v}{d} + \frac{\mu v}{d} = 2\frac{\mu v}{d}[/tex]

[tex]\frac{F}{A} = 2\frac{1.92\times 10^{- 3}\times 0.035}{0.010} = 0.01344 N/m^{2}[/tex]

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