Viscosity — Prelims Strategy

Prelims Strategy

To excel in NEET questions on viscosity, a multi-pronged strategy is essential. Firstly, thoroughly understand the definitions of dynamic viscosity, kinematic viscosity, shear stress, and velocity gradient.

Memorize Newton's Law of Viscosity ($\tau = \eta \frac{dv}{dy}$) and its dimensional formula ($[ML^{-1}T^{-1}]$). Secondly, master Stokes' Law ($F_v = 6\pi\eta r v$) and, most importantly, the derivation and final formula for terminal velocity ($v_t = \frac{2r^2(\rho - \rho_f)g}{9\eta}$).

Pay close attention to the proportionality relationships, such as $v_t \propto r^2$ and $v_t \propto 1/\eta$. Thirdly, understand the contrasting effects of temperature on the viscosity of liquids (decreases) and gases (increases), along with their molecular explanations.

For numerical problems, always convert all given quantities to SI units (e.g., cm to m, Poise to Pa s) before calculation. Be wary of trap options that might use incorrect powers of radius in terminal velocity or confuse the effects of temperature.

Practice a variety of problems, including those involving ratios and comparisons, to build confidence and speed.