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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 ? ......... \(cm/s\)

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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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