If a ball of steel (density ρs=7.8g/cm3) attains a terminal velocity of 10cm/s when falling in a tank of water (coefficient of viscosity, ηw=8.5×10−4Pa-s). Then, its terminal velocity in glycerine (ρg=1.2g/cm3,ηg=13.2Pa-s) would be nearly:
A
1.5×10−5cm/s
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B
6.45×10−4cm/s
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C
6.25×10−4cm/s
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D
1.6×10−5cm/s
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Solution
The correct option is C6.25×10−4cm/s Given: ρs=7.8g cm−3 ρw=1g cm−3 ρg=1.2g cm−3 ηg=13.2Pa-s ηw=8.5×10−4Pa-s
We can say, at terminal velocity condition, net force on the ball must be zero.
i.e W−FB=Fv
where W= Weight FB= Buoyant force Fv= Viscous force
In water: Vb(ρs−ρw)=6πηwrvTw→…(1)
where Vb is the volume of the ball, r its radius and vT is the terminal velocity.