JEE Mains · Physics · STD 11 - Fluid mechanics
- JEE Mains 2026
- Hard
- Numerical
Sixty four rain drops of radius 1 mm each falling down with a terminal velocity of \(10 cm / s\) coalesce to form a bigger drop. The terminal velocity of bigger drop is _________ \(cm / s\).
Pick an option to check your answer.
Answer & Solution
Correct Answer
(C) 160
Step-by-step Solution
Detailed explanation
\(V _{ T }=\frac{2 r ^2 g}{9 n }[\sigma-\rho]\)
\(V _{ T } \propto r ^2\)

64 drop
\(64\left(\frac{4}{3} \pi R_1^3\right)=\frac{4}{3} \pi R_2^3\)
\(R _2=4 R _1\)
\(\frac{\left( V _{ T }\right)_1}{\left(V_{ T }\right)_2}=\left(\frac{ R _1}{ R _2}\right)^2=\left(\frac{1}{4}\right)^2\)
\(\frac{10}{\left(V_{ T }\right)_2}=\frac{1}{16}\)
\(\left( V _{ T }\right)_2=160 cm / sec\)
\(V _{ T } \propto r ^2\)

64 drop
\(64\left(\frac{4}{3} \pi R_1^3\right)=\frac{4}{3} \pi R_2^3\)
\(R _2=4 R _1\)
\(\frac{\left( V _{ T }\right)_1}{\left(V_{ T }\right)_2}=\left(\frac{ R _1}{ R _2}\right)^2=\left(\frac{1}{4}\right)^2\)
\(\frac{10}{\left(V_{ T }\right)_2}=\frac{1}{16}\)
\(\left( V _{ T }\right)_2=160 cm / sec\)
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