Eight drops of equal size are falling through air with steady velocity of 10 cm/sec. If the drops coalesce, what could be its terminal velocity?
A
40 cm/sec
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B
20 cm/sec
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C
30 cm/sec
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D
50 cm/sec
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Solution
The correct option is A 40 cm/sec Terminal velocity, vT=29r2(ρ−σ)gη ∴vT∞r2 ∴vTv′T=r2R2 ...... (i) Now, Volume of the big drop = 8 × volume of each small drop 43πR3=8×43πr3 ⇒R=2r ∴10v′T=r2(2r)2=r24r2 [using equation (i)] ⇒v′T=40cm/s