A solid sphere of diameter 5 mm is rising through an oil at a constant velocity of 1 cm/s. The density and dynamic viscosity of the oil are 850 kg/m³ and 0.7 N-s/m², respectively. Find the density of the material from which the sphere is made.
Given data:
Diameter of sphere D = 5 mm = 0.005 m
Velocity U= 1 cm/s = 0.01 m/s
Density of oil \rho_{o}=850\,{\mathrm{kg}}/{\mathrm{m}}^{3}
Viscosity of oil μ = 0.7 N-s/m²
Reynolds number is found to be
R e=\frac{\rho U D}{\mu}=\frac{850 \times 0.01 \times 0.005}{0.7}=0.06<1
The total drag force is found from Stokes’ formula (Eq. (15.15))
F_{D}=3\pi\mu D U_{\infty} (15.15)
F_D=3 \pi \mu D U
=3\pi\times0.7\times0.005\times0.01=0.0033\,\mathrm{N}
Let {\boldsymbol{\rho}}_{s} be the density of sphere.
\therefore \text { Weight of sphere } W = Density of sphere x g x Volume of sphere
=\rho_s \times g \times \frac{\pi}{6} D^3
=\rho_{s}\times9.81\times{\frac{\pi}{6}}(0.005)^{3}=0.00000064\rho_{s}\,\mathrm{N}
\therefore Buoyant force {\boldsymbol{F}}_{B} = Density of oil × g × Volume of sphere
=\rho_{o}\times g\times{\frac{\pi}{6}}D^{3}=850\times9.81\times{\frac{\pi}{6}}(0.005)^{3}=0.000546~\mathrm{N}
Using Eq. (15.20), we have
W=F_D+F_B
or 0.00000064 \rho_s=0.00033+0.000546
or \rho_s=\frac{0.000876}{0.00000064}=1368.75 \mathrm{~kg} / \mathrm{m}^3