What is the difference between dynamic viscosity and kinematic viscosity?

Dynamic viscosity ($\eta$) is the measure of a fluid's resistance to shear flow, as described by Newton's Law of Viscosity. It quantifies the internal friction. Kinematic viscosity ($\nu$) is the ratio of dynamic viscosity to the fluid's density ($\nu = \eta / \rho$). It represents the fluid's resistance to flow under the influence of gravity. While dynamic viscosity is about the force required to overcome internal friction, kinematic viscosity is more about how quickly momentum diffuses through the fluid. Its SI unit is m$^2$/s, and CGS unit is Stoke (St), where 1 St = $10^{-4}$ m$^2$/s.

Why does the viscosity of liquids decrease with increasing temperature, while that of gases increases?

In liquids, viscosity is primarily due to strong intermolecular cohesive forces. As temperature rises, the kinetic energy of molecules increases, weakening these cohesive forces and making it easier for fluid layers to slide past each other, thus decreasing viscosity. In gases, viscosity arises mainly from the transfer of momentum between molecules in adjacent layers. Higher temperatures mean higher molecular speeds and more frequent, energetic collisions, leading to greater momentum exchange and thus increased resistance to flow, which translates to higher viscosity.

What is terminal velocity and how is it related to viscosity?

Terminal velocity is the constant maximum velocity attained by an object falling through a fluid when the net force acting on it becomes zero. This occurs when the downward gravitational force (weight) is balanced by the upward buoyant force and the upward viscous drag force. Viscosity is directly involved in the viscous drag force, as per Stokes' Law ($F_v = 6\pi\eta r v$). A higher fluid viscosity leads to a greater viscous drag force, which means the object will reach its terminal velocity faster and the terminal velocity itself will be lower for a given object.

Are all fluids Newtonian fluids? What are non-Newtonian fluids?

No, not all fluids are Newtonian. Newtonian fluids are those that obey Newton's Law of Viscosity, meaning their viscosity remains constant regardless of the shear rate (velocity gradient) applied. Water, air, and simple oils are good examples. Non-Newtonian fluids, on the other hand, exhibit a viscosity that changes with the applied shear rate. Examples include ketchup (thins when shaken), blood (viscosity changes with shear), paint, and cornstarch suspensions (thickens under stress). NEET primarily focuses on Newtonian fluids.

What are the practical implications of viscosity in daily life?

Viscosity has numerous practical implications. In automotive engineering, engine oil viscosity is crucial for efficient lubrication across varying temperatures. In medicine, blood viscosity affects circulation and can be an indicator of health issues. In food science, viscosity determines the texture and pourability of products like honey, sauces, and syrups. It's also vital in manufacturing processes involving paints, coatings, and adhesives, ensuring they spread and adhere correctly. Even the speed at which rain falls is influenced by the viscosity of air.

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Version 1Updated 23 Mar 2026

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Viscosity is an intrinsic property of fluids (liquids and gases) that quantifies their resistance to flow or shear deformation. It arises from the internal friction between adjacent layers of the fluid moving at different velocities. When a fluid flows, different layers move relative to one another, and a tangential force is required to maintain this relative motion. This internal resistance, anal…

Quick Summary

Viscosity is a fundamental property of fluids (liquids and gases) that quantifies their internal resistance to flow or shear deformation. It's essentially the 'thickness' of a fluid. This resistance arises from internal friction between adjacent layers of the fluid moving at different velocities.

Newton's Law of Viscosity states that shear stress ( $\tau$ ) is directly proportional to the velocity gradient ( $\frac{dv}{dy}$ ), with the proportionality constant being the coefficient of dynamic viscosity ( $\eta$ ).

The SI unit for viscosity is Pascal-second (Pa s) or N s/m $^2$ , also known as Poiseuille. Its dimensional formula is $[ML^{-1}T^{-1}]$ .

Temperature has opposite effects on the viscosity of liquids and gases: liquid viscosity decreases with increasing temperature due to weakened intermolecular forces, while gas viscosity increases due to enhanced molecular momentum transfer.

Stokes' Law describes the viscous drag force ( $F_v = 6\pi\eta r v$ ) experienced by a sphere moving through a viscous fluid. This law is crucial for understanding terminal velocity, where an object falling through a fluid reaches a constant speed when its weight is balanced by buoyant force and viscous drag.

Viscosity is vital in applications like lubrication, blood flow, and paint formulation.

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Key Concepts

Newton's Law of Viscosity

This law mathematically defines the relationship between the shear stress and the rate of shear deformation…

Stokes' Law and Viscous Drag

Stokes' Law quantifies the viscous drag force experienced by a small, rigid sphere moving through a viscous…

Terminal Velocity Calculation

Terminal velocity ( $v_t$ ) is achieved when the weight of a falling object is balanced by the sum of the…

Viscosity ($\eta$) — Internal resistance to fluid flow.

Newton's Law —

\tau = \eta \frac{dv}{dy}

(Shear stress = Viscosity

\times

Velocity gradient).

Units of $\eta$ — Pa s (SI), Poise (CGS,

1,\text{Poise} = 0.1,\text{Pa s}

). Dimensional formula:

[ML^{-1}T^{-1}]

Temperature Effect — Liquids:

\eta \downarrow

T \uparrow

. Gases:

\eta \uparrow

T \uparrow

Stokes' Law — Viscous drag

F_v = 6\pi\eta r v

for a sphere.

Terminal Velocity —

v_t = \frac{2r^2(\rho - \rho_f)g}{9\eta}

. (Sphere density

\rho

, fluid density

\rho_f

To remember the temperature effect on viscosity: Liquids Lower (viscosity with temp), Gases Grow (viscosity with temp). Think 'LLGG' for 'Liquids Lower, Gases Grow'.

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