Oscillating liquid in U-tube

[kevi555]

Hi,

Just wondering if anyone has any thoughts on how to approach this question:

A U-tube has vertical arms of radii 'r' and '2r', connected by a horizontal tube of length 'l' whose radius increases linearly from r to 2r. The U-tube contains liquid up to a height 'h' in each arm. The liquid is set oscillating, and at a given instant the liquid in the narrower arm is a distance 'y' above the equilibrium level.

Show that the potential energy of the liquid is given by...

U = (5/8)g(rho)(pi)(r^2)(y^2)

'y' is the change in height.

Thanks!

 

[Jhero]

Hello,

This is an interesting problem! To approach this question, you will need to use the principles of fluid mechanics and potential energy. The potential energy of a fluid is given by the formula P = ρgh, where ρ is the density of the fluid, g is the acceleration due to gravity, and h is the height of the fluid column. In this case, we have two fluid columns, one with a radius of r and one with a radius of 2r, connected by a horizontal tube with a varying radius.

To start, let's consider the potential energy of the liquid in the narrower arm. Since the liquid is set oscillating, we can assume that the liquid is at its equilibrium level at some point in time. This means that the height of the liquid column in the narrower arm is y above the equilibrium level. Using the formula for potential energy, we can write the potential energy of this column as P = ρg(r^2)y.

Now, let's consider the potential energy of the liquid in the wider arm. Since the radius of the tube is increasing linearly from r to 2r, we can assume that the height of the liquid column in this arm is also changing linearly from h to 2h. Therefore, the average height of the liquid column in this arm is (h+2h)/2 = (3/2)h. Using the formula for potential energy, we can write the potential energy of this column as P = ρg(2r^2)(3/2)h = 3ρgr^2h.

Now, to find the total potential energy of the liquid in the U-tube, we need to add the potential energies of the two columns together. This gives us:

U = ρg(r^2)y + 3ρgr^2h

Since we know that the total height of the liquid column in each arm is h, we can write h = y + (3/2)h. Substituting this into our equation for potential energy, we get:

U = ρg(r^2)y + 3ρgr^2(y + (3/2)h)

Simplifying this equation gives us:

U = (5/2)ρgr^2y + (9/2)ρgr^3h

Finally, substituting in the value for ρg = (5/8)g and rearr