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Question

If a ball of steel (density ρs=7.8 g/cm3) attains a terminal velocity of 10 cm/s when falling in a tank of water (coefficient of viscosity, ηw=8.5×104 Pa-s). Then, its terminal velocity in glycerine (ρg=1.2 g/cm3,ηg=13.2 Pa-s) would be nearly:

A
1.5×105 cm/s
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B
6.45×104 cm/s
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C
6.25×104 cm/s
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D
1.6×105 cm/s
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Solution

The correct option is C 6.25×104 cm/s
Given: ρs=7.8 g cm3
ρw=1 g cm3
ρg=1.2 g cm3
ηg=13.2 Pa-s
ηw=8.5×104 Pa-s

We can say, at terminal velocity condition, net force on the ball must be zero.
i.e WFB=Fv
where W= Weight
FB= Buoyant force
Fv= Viscous force

In water:
Vb(ρsρw)=6πηwrvTw (1)
where Vb is the volume of the ball, r its radius and vT is the terminal velocity.

In glycerine:
Vb(ρsρg)=6πηgrvTg(2)

(2)(1) gives:

ρsρgρsρw=ηgvTgηwvTw

7.81.27.81=13.2×VTg8.5×104×10
VTg=6.6×8.5×1036.8×13.2
VTg=6.25×104 cm s1

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