Test: Fluid Mechanics Level - 1 - Civil Engineering (CE) MCQ

A fluid offers resistance to change in pressure and volume defined by its Bulk Modulus. It also offers resistance to flow provided by a phenomenon called viscosity.
However, there is no resistance to change in the shape of fluids, unlike that in solid.

Flow of Fluids

• Newton's Law of Motion: This law states that the force acting on a fluid is equal to the change in momentum per unit time. It is applicable to all fluids, including ideal fluids.


• Newton's Law of Viscosity: This law states that the shear stress in a fluid is directly proportional to the velocity gradient perpendicular to the direction of flow. It helps in understanding the behavior of viscous fluids.


• Pascal's Law: Pascal's law states that when there is an increase in pressure at any point in an enclosed fluid, there is an equal increase in pressure at every other point in the fluid. It is essential for understanding the transmission of pressure in fluid systems.


• Continuity Equation: The continuity equation states that the mass flow rate of a fluid is constant in a steady flow system. It helps in understanding the conservation of mass in fluid flow.


• Boundary Layer Theory: The boundary layer theory explains the behavior of fluid flow near solid surfaces. It helps in understanding the formation and characteristics of the boundary layer and its impact on fluid flow.

Therefore, out of the given options, the ideal flow of any fluid must fulfill the Continuity equation (D). The continuity equation ensures that the mass flow rate remains constant in a steady flow system, which is a fundamental principle in fluid dynamics.

Explanation:
The density of water is not constant and varies with temperature. It is generally known that the density of water decreases as the temperature increases. However, at a certain temperature, the density of water reaches its maximum value. This temperature is known as the maximum density temperature or the temperature of maximum density.
Factors affecting the density of water:
1. Temperature: As the temperature of water increases or decreases, its density changes.
2. Pressure: Changes in pressure can also affect the density of water, but for this question, we are only considering the effect of temperature.
Detailed explanation:
1. Water is unique in that its density increases as it cools down from room temperature and reaches a maximum at a certain temperature.
2. At temperatures above this maximum density temperature, the density of water decreases as it cools further.
3. The maximum density of water occurs at approximately 4°C (39.2°F). At this temperature, water molecules are arranged in a way that maximizes the number of hydrogen bonds between them, resulting in a higher density.
4. When water is heated above 4°C, the hydrogen bonds between the water molecules start to break, and the molecules move farther apart. This leads to a decrease in density.
5. As water is cooled below 4°C, the hydrogen bonds become more stable, causing the water molecules to arrange in a way that increases the density. However, when water freezes and turns into ice, its density decreases due to the formation of a crystalline structure.
6. At 0°C, water freezes and forms ice. The density of ice is lower than that of liquid water, which is why ice floats in water.
Conclusion:
The density of water is maximum at 4°C. At this temperature, the water molecules are arranged in a way that maximizes the number of hydrogen bonds between them, resulting in a higher density.

Explanation:
To understand why mercury does not wet glass, we need to examine the property of liquids known as cohesion.
Cohesion:
- Cohesion refers to the attraction between molecules of the same substance.
- It is responsible for the formation of drops and the surface tension of liquids.
- In the case of mercury, the cohesive forces between its own molecules are very strong.
Wetting:
- Wetting is the ability of a liquid to spread and adhere to a solid surface.
- It depends on the balance between adhesive and cohesive forces.
Mercury and Glass:
- Glass is a solid and mercury is a liquid.
- The cohesive forces between mercury molecules are stronger than the adhesive forces between mercury and glass molecules.
- As a result, mercury tends to form droplets on the surface of glass rather than spreading and wetting it.
Adhesion:
- Adhesion refers to the attraction between molecules of different substances.
- In the case of mercury and glass, the adhesive forces are not strong enough to overcome the cohesive forces of mercury.
Conclusion:
- The property of liquid known as cohesion is responsible for mercury not wetting glass.
- The cohesive forces between mercury molecules are stronger than the adhesive forces between mercury and glass molecules, causing mercury to form droplets on the surface of glass.

Explanation:
The property of a fluid by which molecules of different kinds of fluids are attracted to each other is called adhesion. Adhesion is the force of attraction between molecules of different substances. It allows the molecules of one substance to cling to the molecules of another substance. This property is responsible for phenomena such as capillary action, where liquids can rise or be pulled into narrow spaces against the force of gravity.
Key Points:
- Adhesion is the property of a fluid by which molecules of different kinds of fluids are attracted to each other.
- It is the force of attraction between molecules of different substances.
- Adhesion allows the molecules of one substance to cling to the molecules of another substance.
- Adhesion is responsible for phenomena such as capillary action.

Explanation:
The specific weight of a substance is defined as the weight of a unit volume of that substance. In the case of water, the specific weight is 1000 kg/m³.
To determine whether this specific weight holds true under different conditions, we need to analyze the given options:
A: At normal pressure of 760 mm
The specific weight of water does not depend on pressure. It remains constant regardless of the pressure applied. Therefore, the specific weight of water is 1000 kg/m³ at normal pressure of 760 mm.
B: At 4°C temperature
The specific weight of water varies with temperature. However, at 4°C, water reaches its maximum density, and its specific weight is equal to 1000 kg/m³. Therefore, the specific weight of water is 1000 kg/m³ at 4°C temperature.
C: At mean sea level
Mean sea level is defined as the average height of the ocean's surface. The specific weight of water does not depend on the height above or below mean sea level. It remains constant regardless of the altitude. Therefore, the specific weight of water is 1000 kg/m³ at mean sea level.
D: All the above
Based on the explanations given above, we can conclude that the specific weight of water is indeed 1000 kg/m³ under all the mentioned conditions. Therefore, the correct answer is option D.
E: None of the above
This option is incorrect since we have established that the specific weight of water is 1000 kg/m³ under the given conditions.
In summary, the specific weight of water is 1000 kg/m³ at normal pressure of 760 mm, at 4°C temperature, and at mean sea level.

Explanation:
The pressure at a point in a fluid is not the same in all directions when the fluid is both viscous and moving. This can be explained by the concept of viscosity and the flow behavior of fluids.
1. Viscosity: Viscosity refers to the internal friction or resistance to flow within a fluid. It is a measure of the fluid's resistance to shear or deformation.
- Viscous fluids have high internal friction and tend to resist flow, while inviscid fluids have negligible internal friction and flow easily.
- The presence of viscosity in a fluid affects the velocity gradient and the distribution of pressure within the fluid.
2. Flow Behavior: When a fluid is in motion, it experiences different velocities at different points. This variation in velocity leads to a variation in pressure.
- In a viscous fluid, the velocity of the fluid layers near a solid boundary is lower compared to the layers in the center of the flow.
- Due to this variation in velocity, the pressure at a point in the fluid will not be the same in all directions.
3. Viscous and Moving Fluid: When a fluid is both viscous and moving, the pressure at a point will vary in different directions.
- The viscosity of the fluid causes internal friction, which creates different velocities and pressure gradients within the fluid.
- The pressure will be higher in areas where the fluid is moving slower and lower in areas where the fluid is moving faster.
Therefore, the pressure at a point in a fluid will not be the same in all directions when the fluid is both viscous and moving (option E). The presence of viscosity and the flow behavior of the fluid cause variations in pressure within the fluid.

To determine the surface tension of mercury compared to water, we need to understand the concept of surface tension and the factors that affect it.
Surface Tension:
Surface tension is the force acting on the surface of a liquid that causes it to behave like a stretched elastic sheet. It is caused by the cohesive forces between the liquid molecules.
Factors affecting Surface Tension:
The surface tension of a liquid depends on several factors, including:
1. Intermolecular forces: The strength of the attractive forces between the molecules of the liquid.
2. Temperature: As temperature increases, the surface tension generally decreases.
3. Nature of the liquid: Different liquids have different surface tensions due to variations in intermolecular forces.
Comparison between Mercury and Water:
Now let's compare the surface tension of mercury and water based on the factors mentioned above:
1. Intermolecular forces: The intermolecular forces in mercury are stronger than those in water because mercury is a metal and forms metallic bonds. Water, on the other hand, is a polar molecule and forms hydrogen bonds. Metallic bonds are generally stronger than hydrogen bonds, indicating that the intermolecular forces in mercury are stronger.
2. Temperature: The surface tension of a liquid generally decreases with increasing temperature. However, since the comparison is made at normal temperature, this factor does not affect the comparison.
3. Nature of the liquid: As mentioned earlier, mercury has stronger intermolecular forces compared to water. Therefore, the surface tension of mercury is higher than that of water.
Conclusion:
Based on the above analysis, we can conclude that the surface tension of mercury at normal temperature is more compared to that of water. Therefore, option A is the correct answer.

Explanation:
The viscosity of a substance refers to its resistance to flow. It is influenced by factors such as temperature and pressure. In the case of very high pressures, the viscosity of most gases and liquids can show erratic behavior. Here's why:
1. Viscosity and Pressure:
- At low pressures, the viscosity of gases and liquids generally decreases with increasing pressure.
- This is because the increased pressure brings the molecules closer together, reducing the intermolecular spaces and increasing the frequency of molecular collisions.
- These collisions increase the chances of molecular alignment and reduce the resistance to flow, resulting in lower viscosity.
2. Viscosity and High Pressure:
- However, at very high pressures, the behavior of viscosity becomes more complex.
- The increased pressure can cause significant changes in the molecular arrangement and intermolecular forces.
- These changes can lead to variations in the viscosity of gases and liquids, resulting in erratic behavior.
- In some cases, the viscosity may increase due to stronger intermolecular forces or changes in molecular structure.
- In other cases, the viscosity may decrease due to changes in molecular alignment or reduced intermolecular interactions.
3. Factors influencing Viscosity:
- It's important to note that the effect of pressure on viscosity can vary depending on the specific substance and its molecular properties.
- Other factors such as temperature, molecular size, and molecular shape also play a role in determining the viscosity of a substance.
- Therefore, it is not accurate to say that the viscosity of most gases and liquids always increases or decreases at very high pressures.
- Instead, the behavior can be unpredictable and may show erratic changes.
In conclusion, for very high pressures, the viscosity of most gases and liquids can show erratic behavior. It may either increase or decrease depending on factors such as intermolecular forces, molecular alignment, and molecular structure.

Fluid in equilibrium

When a fluid is in equilibrium, it means that it is not accelerating and all the forces acting on it are balanced. In this state, the fluid is at rest or moving with a constant velocity.

Stresses on a fluid

Stress is defined as the force per unit area. When a stress is applied to a fluid, it causes deformation or flow. There are different types of stresses that can be applied to a fluid:

• Tensile stress: It is the stress that tends to elongate or stretch the fluid in one direction.


• Compressive stress: It is the stress that tends to compress or shorten the fluid in one direction.


• Shear stress: It is the stress that tends to deform the fluid by sliding one layer of the fluid over another.


• Bending stress: It is the stress that tends to deform the fluid by bending it.

Fluid in equilibrium and stress

When a fluid is in equilibrium, it means that the forces acting on it are balanced. In this state, the fluid cannot sustain any additional stress because it would cause deformation or flow. Therefore, a fluid in equilibrium cannot sustain any of the above-mentioned stresses:

• A fluid in equilibrium cannot sustain tensile stress because it would cause elongation or stretching of the fluid.


• A fluid in equilibrium cannot sustain compressive stress because it would cause compression or shortening of the fluid.


• A fluid in equilibrium cannot sustain shear stress because it would cause deformation by sliding one layer of the fluid over another.


• A fluid in equilibrium cannot sustain bending stress because it would cause deformation by bending the fluid.

Therefore, the correct answer is C: Shear stress. A fluid in equilibrium cannot sustain shear stress.

Archimedes' Principle:
According to Archimedes' principle, when a body is immersed (partially or fully) in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the body.
Explanation:
When an object is placed in a fluid, such as water or air, it displaces some of the fluid. This displacement creates an upward force known as the buoyant force. The buoyant force is equal to the weight of the fluid displaced by the object.
Now, let's consider the given options:
A: Equal to: The buoyancy force is equal to the weight of the fluid displaced by the body. This option aligns with Archimedes' principle and is the correct answer.
B: Less than: If the buoyancy force is less than the weight of the fluid displaced, the object would sink. This contradicts Archimedes' principle.
C: More than: If the buoyancy force is more than the weight of the fluid displaced, the object would rise or float. This contradicts Archimedes' principle.
D: Unpredictable: The behavior of the buoyancy force is predictable according to Archimedes' principle. It is always equal to the weight of the fluid displaced by the object.
Therefore, the correct answer is A: Equal to. The buoyancy force is equal to the weight of the fluid displaced by the body.

Answer:
The correct answer is option D: all of the above. A balloon lifting in air follows the principles of gravitation, Archimedes principle, and buoyancy. Let's break down each principle and explain how they apply to a balloon lifting in air:
1. Law of Gravitation:
- The law of gravitation states that any two objects in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
- In the case of a balloon lifting in air, the force of gravity acts downwards towards the center of the Earth. This force is responsible for the weight of the balloon and its contents.
2. Archimedes Principle:
- Archimedes principle states that when an object is submerged in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the object.
- In the case of a balloon, the air inside the balloon is less dense than the surrounding air. This difference in density causes the balloon to float in the air, as the upward buoyant force exerted on the balloon is greater than the downward gravitational force.
3. Principle of Buoyancy:
- The principle of buoyancy is closely related to Archimedes principle and refers to the upward force exerted by a fluid on an object immersed in it.
- In the case of a balloon, the air inside the balloon displaces an equal volume of air outside the balloon. This displacement creates an upward buoyant force, which helps in lifting the balloon.
Therefore, a balloon lifting in air follows the principles of gravitation, Archimedes principle, and buoyancy. All of these principles work together to enable the balloon to float and defy gravity.

Surface tension has the units of
The correct answer is C: newtons/m. Here's the detailed explanation:
Definition of Surface Tension:
Surface tension is the force exerted by the surface of a liquid that causes it to behave like a stretched elastic sheet. It is a property of the liquid's surface and is caused by the cohesive forces between the molecules in the liquid.
Units of Surface Tension:
Surface tension is defined as the force per unit length acting perpendicular to an imaginary line drawn on the surface of the liquid. Therefore, the units of surface tension can be determined by dividing the units of force by the units of length.
Units of Force:
The SI unit of force is the newton (N).
Units of Length:
The SI unit of length is the meter (m).
Calculation:
To determine the units of surface tension, we divide the units of force by the units of length:
Surface Tension = Force / Length
Surface Tension = N / m
Therefore, surface tension has the units of newtons per meter (N/m).
Conclusion:
The correct units for surface tension are newtons per meter (N/m), which corresponds to option C.

Surface tension
A: Acts in the plane of the interface normal to any line in the surface.
- Surface tension is a force that acts parallel to the interface between two fluids or between a fluid and a solid surface.
- It acts perpendicular to any line drawn on the surface.
B: Is also known as capillarity.
- Capillarity refers to the phenomenon where liquids rise or fall in narrow tubes or capillary tubes due to surface tension.
C: Is a function of the curvature of the interface.
- Surface tension is directly related to the curvature of the interface between two fluids.
- It is higher where the interface is curved more, and lower where the interface is flat.
D: Decreases with fall in temperature.
- Surface tension generally decreases with a decrease in temperature.
- This is because the intermolecular forces that create surface tension become weaker at lower temperatures.
E: Has no units.
- Surface tension is measured in units of force per unit length, such as N/m or dyn/cm.
- However, it is often referred to as having no units because it is a ratio of forces to distance.
Therefore, the correct answer is A: Acts in the plane of the interface normal to any line in the surface. Surface tension is a force that acts parallel to the interface and perpendicular to any line drawn on the surface.

The stress-strain relation of a Newtonian fluid is linear.
Explanation:

1. A Newtonian fluid is a type of fluid that exhibits a linear relationship between stress and strain.

2. Stress is defined as the force per unit area acting on a material, while strain is the deformation or change in shape of a material in response to stress.

3. In a Newtonian fluid, the stress is directly proportional to the strain, resulting in a linear stress-strain relationship.

4. This linear relationship is described by Hooke's Law, which states that the stress is equal to the elastic modulus multiplied by the strain.

5. The elastic modulus is a measure of the stiffness of a material and is a constant value for a given material.

6. Therefore, the stress-strain relation of a Newtonian fluid is linear, and the fluid behaves in a predictable manner under applied stress.

7. The linear stress-strain relationship is characteristic of materials that exhibit purely elastic behavior, meaning they return to their original shape after the stress is removed.
Conclusion:

The stress-strain relation of a Newtonian fluid is linear, as described by Hooke's Law.

The units of viscosity are given by the formula:
Viscosity = Force/Area × Time
To determine the units of viscosity, we can break down the units of the different components in the formula:
1. Force:
- The SI unit of force is the newton (N).
2. Area:
- The SI unit of area is square meters (m²).
3. Time:
- The SI unit of time is seconds (s).
Combining these units, we can determine the units of viscosity:
Viscosity = Force/Area × Time
= N/m² × s
= N · s/m²
= newton-sec per metre²
Therefore, the correct answer is option C: newton-sec per metre².

Explanation:
When a body is placed over a liquid, whether it sinks or floats depends on the balance between two forces: gravitational force and up-thrust (buoyant force) of the liquid. Here's why the correct answer is option C:
Gravitational force:
- It is the force exerted by the Earth on the body and acts vertically downwards.
- The magnitude of the gravitational force depends on the mass of the body.
Up-thrust force:
- It is the force exerted by the liquid on the body and acts vertically upwards.
- The magnitude of the up-thrust force depends on the volume of the liquid displaced by the body.
- According to Archimedes' principle, the up-thrust force is equal to the weight of the liquid displaced by the body.
Now, let's consider the different scenarios:
If the gravitational force is equal to the up-thrust force:
- The body will be in equilibrium and will neither sink nor float. It will remain submerged at a fixed depth in the liquid.
If the gravitational force is less than the up-thrust force:
- The up-thrust force will be greater than the gravitational force, causing the body to experience a net upward force.
- The body will float on the surface of the liquid.
If the gravitational force is more than the up-thrust force:
- The gravitational force will be greater than the up-thrust force, causing the body to experience a net downward force.
- The body will sink in the liquid.
Therefore, when a body is placed over a liquid, it will sink down if the gravitational force is more than the up-thrust force (option C is correct).

Answer:
The meter that is not associated with viscosity is the Orsat meter.
Explanation:
Viscosity is a measure of a fluid's resistance to flow. Various meters are used to measure viscosity, but the Orsat meter is not one of them. Here is a breakdown of the other meters and their association with viscosity:
1. Redwood meter: It is used to measure the viscosity of petroleum products, such as lubricating oils. It determines the time taken for a certain volume of liquid to flow through a small orifice under specific conditions.
2. Saybolt meter: It is used to measure the viscosity of petroleum products, such as gasoline and diesel fuel. It measures the time taken for a fixed volume of liquid to flow through a calibrated tube under specific conditions.
3. Engler meter: It is used to measure the viscosity of liquids, particularly petroleum products. It determines the time taken for a specific volume of liquid to flow through a calibrated orifice at a specific temperature.
Therefore, the correct answer is D: Orsat as it is not associated with viscosity measurement.

The dimensions of surface tension can be determined by analyzing the equation that relates surface tension to the other fundamental quantities.
The equation for surface tension is:
σ = F / L
where σ is the surface tension, F is the force acting parallel to the surface, and L is the length along which the force is applied.
To determine the dimensions of surface tension, we can analyze the dimensions of each term in the equation:
- Force (F): The dimensions of force are given by [M][L][T]^-2, where [M] represents mass, [L] represents length, and [T] represents time.
- Length (L): The dimensions of length are given by [L].
- Surface tension (σ): The dimensions of surface tension are given by [M][L][T]^-2.
Comparing the dimensions of force and length to the dimensions of surface tension, we find that they are consistent. Therefore, the dimensions of surface tension are:
M^1L^0T^-2
So, the correct answer is A: M^1L^0T^-2.

Manometer and Liquid Combination
A manometer is a device used to measure the pressure of a fluid, often used in scientific experiments or industrial applications. It consists of a U-shaped tube partially filled with a liquid, with one end connected to the system whose pressure is to be measured.
The choice of liquid in a manometer is important as it affects the accuracy and functionality of the device. Here, we will discuss which liquid combination is better for a manometer.
Surface Tension and Liquid Combination
Surface tension is the force that acts on the surface of a liquid and tends to minimize its surface area. It is caused by the cohesive forces between the molecules of the liquid. In the context of a manometer, surface tension plays a role in the behavior of the liquid column in the U-shaped tube.
Analysis of the Options:
A: Higher surface tension
- A liquid with higher surface tension will have stronger cohesive forces between its molecules.
- This results in a more stable and less fluctuating liquid column in the manometer.
- A stable liquid column allows for more accurate pressure measurements.
- Therefore, a liquid combination with higher surface tension is a better choice for a manometer.
B: Lower surface tension
- A liquid with lower surface tension will have weaker cohesive forces between its molecules.
- This can lead to a less stable and more fluctuating liquid column in the manometer.
- A fluctuating liquid column can introduce errors and inaccuracies in pressure measurements.
- Therefore, a liquid combination with lower surface tension is not an ideal choice for a manometer.
C: Surface tension is no criterion
- This option suggests that surface tension does not play a role in determining the better liquid combination for a manometer.
- However, surface tension does indeed affect the stability and accuracy of the liquid column.
- Therefore, this option is incorrect.
D: High density and viscosity
- High density and viscosity are not directly related to the surface tension of a liquid.
- While density and viscosity can also affect the behavior of the liquid column, they are not the main criteria for a better liquid combination in a manometer.
- Therefore, this option is not the correct answer.
E: Low density and viscosity
- Similar to option D, low density and viscosity are not directly related to the surface tension of a liquid.
- While low density and viscosity can have their own advantages, they are not the main criteria for a better liquid combination in a manometer.
- Therefore, this option is not the correct answer.
Conclusion:
Based on the analysis, option A, which suggests a higher surface tension, is the correct answer. A liquid combination with higher surface tension provides a more stable and accurate liquid column in a manometer, resulting in more reliable pressure measurements.

Explanation:
The property of fluid that offers resistance to shear is called viscosity. Viscosity is a measure of the internal friction of a fluid and determines how easily it flows. It is a property of both liquids and gases.
To further elaborate:
Surface tension:
- Surface tension is the property of a liquid that allows it to resist an external force and minimize its surface area.
- Surface tension is not related to the resistance to shear.
Adhesion:
- Adhesion is the attraction between molecules of different substances.
- Adhesion does not directly relate to the resistance to shear.
Cohesion:
- Cohesion is the attraction between molecules of the same substance.
- Cohesion does not directly relate to the resistance to shear.
Viscosity:
- Viscosity is the property of a fluid that determines its resistance to flow.
- It is a measure of the internal friction between fluid layers as they slide past each other.
- Viscosity depends on the type of fluid and the temperature.
- Liquids with high viscosity flow slowly, while liquids with low viscosity flow easily.
Conclusion:
The property of fluid by virtue of which it offers resistance to shear is called viscosity. Surface tension, adhesion, and cohesion are not directly related to the resistance to shear.

Explanation:
To determine the density of water at different temperatures, we need to consider the temperature-dependence of water's density.
Density of water at 0 °C:
- Water has a maximum density at 4 °C. At this temperature, the density of water is 1000 kg/m^3.
- As the temperature decreases from 4 °C to 0 °C, the density of water slightly increases. Therefore, the density of water at 0 °C is also 1000 kg/m^3.
Density of water at 0 K:
- 0 K is absolute zero, the lowest possible temperature.
- At absolute zero, the molecules in water do not have any kinetic energy and are in their lowest energy state.
- At this temperature, the water molecules would be motionless and the density of water would be zero. Therefore, the density of water at 0 K is not 1000 kg/m^3.
Density of water at 4 °C:
- As mentioned earlier, water has a maximum density at 4 °C, which is 1000 kg/m^3. This is due to the unique hydrogen bonding arrangement of water molecules.
- Above or below 4 °C, the density of water decreases.
- Therefore, the density of water at 4 °C is indeed 1000 kg/m^3.
Density of water at 20 °C:
- As the temperature increases from 4 °C to 20 °C, the density of water decreases.
- At 20 °C, the density of water is approximately 998 kg/m^3, which is slightly lower than the density at 4 °C.
- Therefore, the density of water at 20 °C is not 1000 kg/m^3.
Density of water at all temperatures:
- Water's density varies with temperature, and it is not constant for all temperatures.
- The density of water reaches its maximum at 4 °C and decreases as the temperature deviates from this value.
- Therefore, the density of water is not constant at all temperatures.
Conclusion:
- The correct answer is C: 4 °C. The density of water is 1000 kg/m^3 at this temperature.

The ratio of absolute viscosity to mass density is known as:
C: Kinematic viscosity
Explanation:
The ratio of absolute viscosity to mass density is known as kinematic viscosity. It is a measure of the resistance of a fluid to flow under an applied force. Here's a detailed explanation of the terms mentioned in the options:
- Specific viscosity: This term is not correct in this context. Specific viscosity refers to the ratio of the absolute viscosity of a fluid to the viscosity of a standard fluid at the same temperature and pressure.

- Viscosity index: This term is also not correct in this context. Viscosity index is a measure of how the viscosity of a fluid changes with temperature. It is used to characterize the viscosity-temperature relationship of lubricating oils.

- Kinematic viscosity: This is the correct term in this context. Kinematic viscosity is the ratio of the absolute viscosity of a fluid to its mass density. It is often denoted by the symbol "ν" and has units of square meters per second (m²/s).

- Coefficient of viscosity: This term is not correct in this context. The coefficient of viscosity is another name for absolute viscosity, which is a measure of a fluid's internal resistance to flow.

- Coefficient of compressibility: This term is not correct in this context. The coefficient of compressibility is a measure of how much a fluid's volume changes in response to changes in pressure.
In summary, the correct answer is C: Kinematic viscosity, as it represents the ratio of absolute viscosity to mass density.