MHT CET · Physics · Mechanical Properties of Fluids

A body of density ' \(\rho\) ' is dropped from rest at a height ' \(h\) ' into a lake of density ' \(\sigma\) ' \((\sigma>\rho)\). The maximum depth to which the body sinks before returning to float on the surface is (neglect air dissipative forces)

A \(\frac{\mathrm{h} \rho}{(\sigma-\rho)}\)
B \(\frac{h \rho}{(\sigma+\rho)}\)
C \(\frac{h \rho}{(\rho-\sigma)}\)
D \(\frac{2 h \rho}{(\sigma-\rho)}\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\frac{h \rho}{(\rho-\sigma)}\)

Step-by-step Solution

Detailed explanation

Initial velocity of the ball \(=\sqrt{2 \text { gh }}\)
Upward force:
\(
\begin{aligned}
& F=\sigma V g-\rho V g \\
& (\rho V) a=V g(\sigma-\rho) \\
\therefore \quad & a=\frac{g(\sigma-\rho)}{\rho}
\end{aligned}
\)
Final velocity is zero when it sinks.
\(
\begin{aligned}
\therefore \quad & v^2-u^2=2 \text { as } \\
& 0-(\sqrt{2 g h})^2=2 \frac{g(\sigma-\rho)}{\rho} H \\
& H=\frac{h \rho}{(\rho-\sigma)}
\end{aligned}
\)

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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