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Question: Answered & Verified by Expert
A given shaped glass tube having uniform cross section is filled with water and is mounted on a rotatable shaft as shown in figure. If the tube is rotated with a constant angular velocity $\omega$ when

PhysicsMechanical Properties of FluidsAIIMSAIIMS 2005
Options:
  • A water levels in both sections $A$ and $B$ go up
  • B water level in section $A$ goes up and that in $B$ comes down
  • C water level in section $A$ comes down and that in $B$ it goes up
  • D water levels remain same in both sections.
Solution:
1825 Upvotes Verified Answer
The correct answer is: water levels in both sections $A$ and $B$ go up
Due to rotation about the axis $O O^{\prime}$, a force is acting on the liquid column $A B$ outwards, pushing up the liquid column on the right. This centrifugal force acting on the small element $d x$ of area of cross-section $d x$, distant $x$ from the axis is
$$
d m \omega^2 x=A d x \rho \omega^2 x
$$
The total force due to column of length $L(=A B)$ is
$$
\int_0^L A \rho \omega^2 \cdot x d x=A \rho \omega^2 \frac{L^2}{2}
$$




Pressure at $B=$ atmospheric pressure +
$$
h_1 \rho g+\frac{A \rho \omega^2}{A} \frac{L^2}{2}
$$
Pressure at $B$ due to liquid column on the right hand side $=$ atmospheric pressure $+h_2 \rho g$
Net pressure due to the left hand side $=$ pressure at $B$ due to liquid column
$$
\begin{aligned}
& \quad \text { atm. pressure }+h_1 \rho g+\rho \omega^2 \frac{L^2}{2}=\text { atm. pressure }+h_2 \rho g \\
& \therefore \quad h_2-h_1=H_0=\frac{\omega^2 L^2}{2 g} .
\end{aligned}
$$

$$
\text { Here one has }
$$


$$
H_2=\frac{\omega^2 L_2^2}{2 g}, \quad H_1=\frac{\omega^2 L_1^2}{2 g}
$$
As $\omega$ is the same, and $L_2>L_1, H_2>H_1$. But both will go up.

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