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  1. The terminal velocity vr of a small steel ball of radius r falling under gravity through a column of a viscous liquid of coefficient of viscosity η depends on mass of the ball m, acceleration due to gravity g, coefficient of viscosity η and radius r. Which of the following relations is dimensionally correct ?​​​
    1. vr
      mgr
      η
    2. vr ∝ mg η r
    3. vr
      mg
    4. vr
      η mg
      r
Correct Option: C

Note that according to Stoke's law
6π η rvr = mg
Hence, the valid relation is vr ∝ mg / rη



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