Q: A spherical body of mass m and radius r is allowed to fall in a medium of viscosity η . The time in which the velocity of the body increases from zero to 0.65 times the terminal velocity (υ) is called time constant t . the dimensions of t are

(a) $\frac{mr^2}{6\pi \eta}$

(b) $ \sqrt{\frac{6 \pi m r \eta}{g^2}}$

(c) $\frac{m}{6 \pi \eta r v} $

(d) None of these

Ans: (d)