MHT-CET Physics Crash Course “Viscosity and Velocity Gradient” Class 12

“Viscosity and Velocity Gradient” MHT-CET Class 12 Physics Crash Course | Chapter Mechanical Properties of Fluids

In this session G.S. Khairnar discussed the theory and MCQ’s of “Viscosity and Velocity Gradient”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.

Viscosity and Velocity Gradient:-

Hello dear students, in previous articles we had learnt the motion of fluid. The motion of liquid is classified is into two types i.e. streamline flow and turbulent flow.

Streamline flow:

  1. The flow of liquid in which velocity of individual layer remains constant throughout the motion is called as streamline flow or laminar flow.

Turbulent flow:

  1. The flow of liquid in which velocity of individual layer do not remains constant throughout the motion is called as turbulent flow.

In this article we are going to understand the friction in the layers of liquid/fluid which gives rise the concept of viscosity.

Let’s discuss the concept of viscosity……………!

We are familiar that there is the opposition to relative between the two solid surfaces in contact is known as friction. This friction causes due to interlocking of hills and dales present on the surface of substance in contact. Frictional forces are self adjusting forces. Like solids, liquids also possess the property friction due to presence of cohesive forces between the adjacent atoms/molecules of fluid. The friction in fluids is known as viscosity.

The property by virtue of which there is opposition to relative motion between the layers of liquid is called as viscosity.

  1. The liquid with more viscosity flows slowly than that of liquid with low value of viscosity. Eg. honey and water
  2. Viscosity of liquid decreases with increase in temperature of liquid.
  3. The fluid in which all layers moves with same speed is known as non viscous fluid. (e.g. helium at 2000 K)

Velocity gradient:

We know that the liquid is consists of different layers which travels with different velocities throughout the flow. The velocity of topmost layer is always high as this layer has opposition from the air above it and a layer of liquid present below it. As the distance of layers changes with respect to the layer at the bottom, velocity also changes. The relation between the velocities of layers and the distance of that layer from the lowest layer is known as velocity gradient.

Consider the flow of liquid as shown in fig above in which different layers of liquid with different velocities. Let ‘v’ be the velocity of any layer at distance ‘x’ from the bottom. Suppose that the velocity of layer above it is slightly more by value ‘dv’

Hence the velocity of that layer will be v+ dv and distance from the bottom will be x+dx

∴ change in velocity=v+dv-v=dv

∴ change in distance=x+dx-x=dx

This value will be same for the layers below it. Therefore we can say that the ratio of change in velocity per unit change in distance of the layer is constant for the given liquid known as velocity gradient.

Viscosity and Velocity Gradient:-

Let’s solve some MCQ’s….!

Q.1) The opposition to the relative motion between layers of liquid is known as ……

a) friction

b) surface tension

c) viscosity

d) none of these


c) viscosity

Q.2) Ratio of rate of change of velocity of layers of liquid per unit distance from bottom is termed as ……

a) viscous force

b) velocity gradient

c) viscosity

d) none of these


b) velocity gradient

Q.3) The fluid in which all the layers moves with constant speed throughout is known as

a) non viscous fluid

b) viscous fluid

c) ideal fluid

d) all of these


a) non viscous fluid

Q.4) SI unit of velocity gradient is

a) per second

b) per meter

c) per second2

d) per meter2


a) per second

Q.5) The difference in velocity of adjacent layers of liquid is 2.5 cm/s if the corresponding difference of distance of the layers from the bottom is 1.25 mm. Velocity gradient of fluid is….

a) 0.2 /sec

b) 2 /sec

c) 20 /sec

d) 200/sec


a) per second


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