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JEE Mains · Physics · STD 11 - Fluid mechanics
- JEE Mains 2013
- Hard
- Numerical
In an experiment, a small steel ball falls through a Iiquid at a constant speed of \(10\, cm/s\). If the steel ball is pulled upward with a force equal to twice its effective weight, how fast will it move upward ? ......... \(cm/s\)
Pick an option to check your answer.
Answer & Solution
Correct Answer
(C) \(10\)
Step-by-step Solution
Detailed explanation
Weight of the body
\(W = mg = \frac{4}{3}\pi {r^3}\rho g\)
\(T = \frac{4}{3}\pi {r^3}\sigma g\)
\(and\,F = 6\pi \eta vr\)
When the body attains terminal velocity net force acting on the body is zero \(i.e.,\)
\(W - T - F = 0\)
And terminal velocity \(v = \frac{2}{9}\frac{{{r^2}\left( {\rho - \sigma } \right)}}{\eta }g\)
As in case of upward motion upward force is twice its effective weight, therefore, it will move with same speed \(10cm/s\)
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