The unit of viscosity is a. dynes b. newton c. gram d. poise

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Q. The unit of viscosity is a. dynes b. newton c. gram d. poise

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Concept understanding — Viscosity

Imagine a thick, golden stream of honey being poured from a jar. It moves slowly, in a smooth, lazy ribbon. Now picture water from a tap — it rushes out, splashes, and flows freely. The difference you see is viscosity. It is the fluid's internal friction, its reluctance to let one layer slide past another. Honey has high viscosity; water has low viscosity.

At the microscopic level, a fluid is made of molecules that are constantly jostling and colliding. In a more viscous fluid, the molecules are either larger, more tangled, or have stronger attractive forces between them. When you try to make one layer of the fluid move relative to the layer next to it, these molecular interactions resist that motion. That resistance is what you feel as "thickness" or "stickiness."

Now, let's make this precise. Consider a fluid trapped between two large parallel plates, one stationary and one moving at a steady speed. The layer of fluid right next to the moving plate sticks to it and moves with the same speed. The layer next to the stationary plate stays at rest. In between, the speed of the fluid changes gradually from zero to the plate's speed. This is called a velocity gradient.

The force required to keep the top plate moving is the viscous force. Experiments show that this force $F$ is proportional to two things: the area $A$ of the plate in contact with the fluid, and the velocity gradient $\frac{dv}{dx}$ (how quickly the speed changes as you move perpendicular to the flow). The constant of proportionality is called the coefficient of viscosity, denoted by $\eta$ (eta).

F = \eta \, A \, \frac{dv}{dx}

Here, $F$ is the viscous force that opposes relative motion between adjacent fluid layers. $A$ is the area of contact between those layers. $\frac{dv}{dx}$ is the velocity gradient — the rate at which velocity changes with distance perpendicular to the flow. And $\eta$ is the viscosity of the fluid, a property that depends on the fluid itself and its temperature (heating a fluid usually lowers its viscosity).

Watch out

Do not confuse viscosity with density. A dense fluid (like mercury) can flow easily, while a less dense fluid (like cold treacle) can be very viscous. They measure completely different things: density is mass per volume; viscosity is resistance to flow.

So, the intuitive picture of "thickness" is captured mathematically by this simple law. The larger $\eta$ is, the larger the force needed to slide one layer past another at the same speed. That is why honey pours slowly — its high $\eta$ means a small velocity gradient already produces a large resisting force, so it takes more effort (gravity, in this case) to make it flow.

Viscosity and its coefficient are core NCERT/CBSE Class 11 Physics topics under mechanical properties of fluids, and "viscosity: definition, formula & real-world examples" is a frequently searched query during exam preparation. It's also a foundational concept for JEE Main and NEET questions on Stokes' law and terminal velocity.

Step 1. Viscosity's SI unit is N s m-2, but in the older CGS system it is measured in poise (1 poise = 1 g cm-1 s-1).

Step 2. Of the given options, dynes is a unit of force, newton is also a unit of force (SI), and gram is a unit of mass -- none of these are units of viscosity.

Step 3. Poise is specifically the standard CGS unit used for the coefficient of viscosity.

✓Final answer

d. poise

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 19 on this concept.

CBSE 2026Set ANNUAL1 markMCQ
$Q. Which among the following forces is developed due to resistance of a fluid flow?$
(a) Viscous force
(b) Inertial force
(c) Gravitational force
(d) Pressure force
›Reveal solutionSolution

Viscosity is the fluid property responsible for a dragging/frictional force between adjacent layers of fluid moving at different speeds -- this is called the viscous force, and it directly resists flow.

Viscosity arises due to internal friction between adjacent layers of a fluid that are moving with different velocities relative to each other. The tangential force per unit area needed to maintain this relative motion (or the resisting force it produces) is the viscous force, given by Newton's law of viscosity:

F = eta A (dv/dx)

where eta is the coefficient of viscosity. This is precisely the force that resists/opposes fluid flow (e.g., it is why thick fluids like honey flow more slowly than water). Inertial force resists changes in motion generally (not specific to fluid flow resistance), gravitational force is unrelated to flow resistance, and pressure force pushes fluid rather than resisting its internal flow.

✓Final answer
$(a) Viscous force.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. What is the dimensional formula of the coefficient of viscosity?$
(a) ML^-2T^-1
(b) ML^-1T^-2
(c) ML^-1T^-1
(d) ML^-2T^-2
›Reveal solutionSolution

The coefficient of viscosity has dimensional formula ML^-1T^-1 (SI unit: Pa·s = kg m^-1 s^-1).

From Newton's law of viscous flow, the viscous force between two layers of fluid is F = η A (dv/dx), where A is the area of contact and dv/dx is the velocity gradient. Solving for η: η = F / [A · (dv/dx)]. Now substitute dimensions: [F] = MLT^-2, [A] = L^2, [dv/dx] = (LT^-1)/L = T^-1. So [η] = MLT^-2 / (L^2 · T^-1) = MLT^-2 · L^-2 · T = ML^-1T^-1.

✓Final answer
$The correct option is (c) ML^-1T^-1.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. The dimensions of the coefficient of viscosity are$
(a) [MLT^-1]
(b) [ML^2T^-2]
(c) [M^-1LT^-1]
(d) [ML^-1T^-1]
›Reveal solutionSolution
Coefficient of viscosity has dimensions [ML^-1 T^-1]. Answer (D).

Newton's law of viscosity: F = eta A (dv/dx), so eta = F/(A (dv/dx)).

Dimensions:

F -> [MLT^-2]

A -> [L^2]

dv/dx (velocity gradient) -> [LT^-1]/[L] = [T^-1]

eta = [MLT^-2]/([L^2][T^-1]) = [MLT^-2] / [L^2 T^-1] = [M L^-1 T^-1].

✓Final answer
$(D) [ML^-1 T^-1].$
CBSE 2026Set ANNUAL1 markMCQ
$Q. Viscous force$
(a) is a non-conservative force
(b) depends on the velocity
(c) acts opposite to the fluid flow
(d) all of these
›Reveal solutionSolution
All three properties hold, so the answer is (D) all of these.

Viscous force between fluid layers (F = eta A dv/dx):

Non-conservative: it converts mechanical energy into heat and depends on the path, so it is non-conservative.

Velocity-dependent: it is proportional to the velocity gradient (relative velocity of the layers).

Opposes flow: like friction, it acts opposite to the direction of relative motion of the fluid.

All statements are correct.

✓Final answer
$(D) all of these.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. With increase of temperature viscosity of maximum liquids (A) decreases (B) increases (C) remains same (D) none of these$
›Reveal solutionSolution

Viscosity of most liquids decreases as temperature increases.

Viscosity in a liquid arises mainly from intermolecular cohesive forces between neighbouring layers of the liquid. As temperature rises, molecules gain kinetic energy, move about more freely, and the average intermolecular spacing increases — weakening the cohesive forces that resist relative motion between layers. This reduces the liquid's viscosity. (This is the opposite trend to gases, where viscosity arises from momentum transfer between molecules and actually increases with temperature.)

✓Final answer
$(A) decreases.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. The frictional resistance for fluids in motion is$
(a) inversely proportional to the square of the surface area of contact
(b) inversely proportional to the surface area of contact
(c) proportional to the square of the surface area of contact
(d) proportional to the surface area of contact
›Reveal solutionSolution

Newton's law of viscosity gives the viscous force as F = eta A (dv/dx) -- directly proportional to the area of contact between the fluid layers, not inversely.

For a fluid in motion, adjacent layers moving at different speeds experience an internal frictional (viscous) force opposing relative sliding, given by Newton's law of viscosity:

F = eta * A * (dv/dx)

where eta is the coefficient of viscosity, A is the area of contact between the layers, and dv/dx is the velocity gradient.

This shows F is directly proportional to A -- a larger contact area means a larger viscous (frictional) resistance force.

✓Final answer
$(d) proportional to the surface area of contact.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. Match the following - Column A item: Coefficient of viscosity. Pick the matching relation from Column B.$
(a) F.V (Force . Velocity)
(b) T is proportional to sqrt(l)
(c) eta is proportional to 1/(dv/dx)
(d) mu_s = tan(theta)
(e) Y is proportional to 1/l
(f) v^2/r
›Reveal solutionSolution

Newton's law of viscous flow gives the coefficient of viscosity eta in terms of the velocity gradient dv/dx.

By Newton's law of viscosity, the tangential (shearing) stress F/A between adjacent fluid layers is proportional to the velocity gradient perpendicular to the flow: F/A = eta (dv/dx), so eta = (F/A)/(dv/dx). For a fixed shearing stress, a fluid with larger viscosity eta produces a smaller velocity gradient dv/dx between its layers — i.e., eta is inversely related to dv/dx, matching the printed relation eta proportional to 1/(dv/dx).

✓Final answer
$Coefficient of viscosity matches option (c): eta is proportional to 1/(dv/dx).$
CBSE 2024Set ANNUAL1 markMCQ
$Q. If two small spheres of radii r/2 and r are moving in fluid with equal constant velocity, then the ratio of viscous force acting on them will be (A) 1:2 (B) 2:1 (C) 1:4 (D) 4:1$
›Reveal solutionSolution

Viscous force ratio for radii $r/2$ and $r$ is $1:2$ , since $F\propto r$ .

Stokes' law: $F=6\pi\eta rv$ , so at the same velocity $v$ and viscosity $\eta$ , $F\propto r$ .

For the two spheres: $\dfrac{F_1}{F_2}=\dfrac{r/2}{r}=\dfrac12$ , i.e. $F_1:F_2=1:2$ .

✓Final answer
$(A) 1:2.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Dimension of coefficient of viscosity is (A) [ML^-1T^-1] (B) [MLT] (C) [MLT^-2] (D) [ML^-2T^-2]$
›Reveal solutionSolution

Coefficient of viscosity has dimension $[ML^{-1}T^{-1}]$ .

From $F=\eta A\dfrac{dv}{dx}$ : $\eta=\dfrac{F}{A\,(dv/dx)}$ .

Dimensions: $[F]=[MLT^{-2}]$ , $[A]=[L^2]$ , $[dv/dx]=[LT^{-1}]/[L]=[T^{-1}]$ .

$[\eta]=\dfrac{[MLT^{-2}]}{[L^2][T^{-1}]}=[ML^{-1}T^{-1}]$ .

✓Final answer
$[ML^{-1}T^{-1}]$
CBSE 2024Set SET-AP55001 mark
$Q. The viscosity of gases ________ on increasing temperature.$
›Reveal solutionSolution

Gas viscosity increases with temperature (opposite to the trend in liquids), because it arises from momentum transfer between molecules moving between adjacent layers, which intensifies as the molecules move faster.

In a gas, viscosity is caused by molecules diffusing between layers moving at different velocities relative to each other, carrying momentum with them and thereby exerting a dragging (frictional) effect between the layers. As temperature increases, molecular speeds increase (kinetic theory: average speed ∝ √T), so this momentum-transfer mechanism becomes more effective, and viscosity increases — roughly η ∝ √T.

This is the opposite of what happens in liquids, where viscosity is dominated by intermolecular cohesive forces that weaken as temperature rises, so liquid viscosity DEcreases with temperature.

✓Final answer
$The viscosity of gases increases on increasing temperature.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. With an increase in temperature, the viscosity of liquid and gas, respectively will:$
(a) decrease and increase
(b) increase and increase
(c) decrease and decrease
(d) increase and decrease
›Reveal solutionSolution

Liquid viscosity is dominated by intermolecular cohesive forces, which weaken as temperature rises, so liquid viscosity decreases with temperature. Gas viscosity is dominated by molecular momentum transfer, which increases with temperature, so gas viscosity increases with temperature.

In a liquid, viscosity is caused mainly by the cohesive forces between closely-packed molecules. As temperature increases, molecules gain kinetic energy and move more freely, weakening these cohesive forces, so the liquid flows more easily — viscosity decreases.

In a gas, molecules are far apart and viscosity arises from the transfer of momentum as faster molecules diffuse into slower-moving layers. As temperature increases, molecular speeds increase, so more momentum is transferred between layers per unit time, and viscosity increases.

✓Final answer
$With an increase in temperature, the viscosity of a liquid decreases and that of a gas increases — option (a).$
CBSE 2023Set ANNUAL1 markMCQ
$Q. The dimensional formula for coefficient of viscosity is:$
(a) ML^-2T^-2
(b) MLT^-2
(c) ML^-1T^-2
(d) ML^-1T^-1
›Reveal solutionSolution
Viscosity is defined by Newton's law of viscous flow, F = eta A (dv/dx); solving for eta and substituting dimensions gives ML^-1T^-1.

By Newton's law of viscosity, the viscous force between two liquid layers is

F = eta A (dv/dx)

where A is the area of contact and dv/dx is the velocity gradient (rate of change of velocity with distance).

Rearranging,

eta = F / [A x (dv/dx)]

Now write the dimensions of each quantity:

Force F: [MLT^-2]

Area A: [L^2]

Velocity gradient dv/dx: [LT^-1]/[L] = [T^-1]

So,

[eta] = [MLT^-2] / ([L^2][T^-1]) = M L^(1-2) T^(-2+1) = ML^-1T^-1

✓Final answer
$The correct option is (d) ML^-1T^-1.$

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Q.The unit of viscosity is a. dynes b. newton c. gram d. poise

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