Physics And Measurement Question 85

Question: A small steel ball of radius $ r $ is allowed to fall under gravity through a column of a viscous liquid of coefficient of viscosity $ \eta $ . After some time the velocity of the ball attains a constant value known as terminal velocity $ v _{T} $ . The terminal velocity depends on (i) the mass of the ball $ m $ , (ii) $ \eta $ , (iii) $ r $ and (iv) acceleration due to gravity $ g $ . Which of the following relations is dimensionally correct

[CBSE PMT 1992; NCERT 1983; MP PMT 2001]

Options:

A) $ v _{T}\propto \frac{mg}{\eta r} $

B) $ v _{T}\propto \frac{\eta r}{mg} $

C) $ v _{T}\propto \eta rmg $

D) $ v _{T}\propto \frac{mgr}{\eta } $

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Answer:

Correct Answer: A

Solution:

By substituting dimension of each quantity in R.H.S. of option

we get $ [ \frac{mg}{\eta r} ]\ =\ [ \frac{M\times L{{T}^{-2}}}{M{{L}^{-1}}{{T}^{-1}}\times L} ] $ = $ [L{{T}^{-1}}] $ .

This option gives the dimension of velocity.



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