Viscosity is the property of a fluid due to which it opposes relative motion between its different layers; the relevant rate is the velocity gradient dv/dx.

• Use the word 'relative' carefully: if both layers move, take the velocity difference, not just the speed of one layer.
• High viscosity means stronger resistance to shear flow, so honey and glycerin oppose layer sliding more than water or air.
• Common NEET trap: treating viscosity as resistance against the whole fluid bulk instead of resistance between adjacent layers.

Newton's law of viscous force is F = -eta A(dv/dx); hence eta = F / [A(dv/dx)], with SI unit N s m^-2 and dimension [M L^-1 T^-1].

• Force is directly proportional to contact area A, so doubling plate area doubles the viscous force if eta and dv/dx stay fixed.
• The minus sign shows opposition to relative motion; it tells direction, not a negative magnitude of viscosity.
• Common NEET trap: mixing poise and poiseuille or forgetting that 1 poiseuille = 10 poise while converting answer choices.

With increase in temperature, the coefficient of viscosity of liquids decreases but that of gases increases; the textbook also lists Andrade's empirical relation for temperature dependence.

• For liquids, heating weakens effective cohesive hold between neighboring layers, so they slide more easily and viscosity falls.
• For gases, heating increases random molecular motion and collision activity, so momentum transfer between layers rises and viscosity increases.
• Common NEET trap: applying the liquid trend to air or any gas because the words 'heated fluid flows faster' sound similar.