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Fluid dynamics - class-XI

Description: fluid dynamics
Number of Questions: 23
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Tags: physics option b: engineering physics
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Choose the wrong statement.

  1. Viscous force always supports the motion.

  2. A denser fluid always more viscous than a less dense fluid.

  3. Any object cannot have a velocity after a sufficient long time more than its terminal velocity in a given fluid.

  4. If a body is at rest in a fluid then the viscous force on it is zero.


Correct Option: A
Explanation:

Viscous force is a frictional force which opposes the relative motion between liquid molecules. Terminal velocity is the velocity attained by the particle when net force on it becomes zero the body cannot have more velocity than terminal velocity in respective fluid.

The velocity of falling rain drops attains limited value because of :

  1. surface tension

  2. upthrust due to air

  3. viscous force exerted by air

  4. air current


Correct Option: C
Explanation:

Gravity does cause things to fall with increasing speed, but as they speed up viscous force by air increases increases. Since according to stokes law $F _r \propto v$.  Eventually the acceleration stops and the raindrop reaches a constant terminal velocity.

Clouds appear to float in air due to:

  1. low density.

  2. air current.

  3. viscosity of air.

  4. buoyancy.


Correct Option: C
Explanation:

Viscosity of air is responsible for floating of clouds is air

The quality of fountain-pen ink depends largely on:

  1. Surface tension of the liquid

  2. Viscosity of ink

  3. impurities in ink

  4. Density of ink


Correct Option: B
Explanation:

The quality of fountain-pen ink depends largely on viscosity of ink because as the viscosity of ink is more, it requires hard pressure on the pen to write and as the viscosity of ink is less  we can write smoothly.

Two identical lead shots are dropped at the same time in two glass jars containing water and glycerine. The lead shot dropped in glycerine descends slowly because:

  1. Viscous force is more in water than in glycerine

  2. Viscous force is more in glycerine than in water

  3. Surface tension is more in water

  4. Surface tension is more in glycerine


Correct Option: B
Explanation:

Glycerine viscosity is 950 cP and water viscosity is 0.89 cP. We can see that glycerine has high viscosity. High viscous force of glycerine increases the time of drop of lead shot than in water.

After the storm, the sea water waves subside due to:

  1. Surface tension of sea-water

  2. Disapperance of heavy currents

  3. The viscosity of sea water

  4. Gravitational pull of the storm


Correct Option: C
Explanation:

After the storm, the sea water waves subside due to the viscosity of sea water. Since viscosity's main cause is cohesive force which acts in intermolecular particles. So, sea water waves subside due to these cohesive force between water molecules.

A good lubricant must have:

  1. high viscosity

  2. low viscosity

  3. high density

  4. low surface tension


Correct Option: A
Explanation:

Good lubricant must have high viscosity. So, that machine parts can do work better. Higher the viscosity of the lubricants the greater is its ability to provide boundary lubrication, which means a boundary between moving parts. 

When stirring of a liquid is stopped, the liquid comes to rest due to:

  1. surface tension

  2. gravity

  3. viscosity

  4. buoyancy


Correct Option: C
Explanation:

When stirring of a liquid is stopped, the liquid comes to rest due to viscosity of liquid. While stirring fluid at the axis moves faster in circular path whereas the fluid in contact with the walls does not move. So, the viscous drag force acting between the fluid layers tries to restrict the relative motion of them. So, by viscous drag force liquid comes to rest. 

Coefficient of viscosity of a gas:

  1. increases with increase of temperature.

  2. decreases with increase of temperature.

  3. remains constant with increase of temperature.

  4. may increase or decrease with increase of temperature.


Correct Option: A
Explanation:

With increase in temperature, the molecule of gas carry more momentum and collide more often, that means more viscosity. So, coefficient of viscosity of a gas increases  with increase of temperature.

Viscosity of water at constant temperature is:

  1. more in deep water

  2. more in shallow waters

  3. less in deep water

  4. same in both deep water and shallow waters


Correct Option: A
Explanation:

In deep water temperature decreases, viscosity of water increases with decrease in temperature. So, viscosity of water at constant temperature is more in deep water as temperature is less in deep waters.

Hot syrup flows faster because:

  1. Surface tension increases when temperature is increased.

  2. Viscosity decreases when temperature is increased.

  3. Viscosity increases when temperature is increased.

  4. Surface tension decreases when temperature is increased.


Correct Option: B
Explanation:

Hot syrup is liquid at higher temperature. We know that as temperature increases viscosity decreases for liquids. So, hot syrup flows faster as viscosity decreases.

Assertion : Water flows faster than honey.
Reason: The coefficient of viscosity of water is less than honey.

  1. Both assertion and reason are true and reason is correct explanation of assertion.

  2. Both assertion and reason are true but reason is not the correct explanation of assertion.

  3. Assertion is true but reason is false.

  4. Both assertion and reason are false.


Correct Option: A
Explanation:

Velocity is inversely proportional to coefficient of viscosity, $\eta$ of water is less than that of honey.
Hence, water flows faster than honey.

With increase in temperature, the viscosity of:

  1. gases decreases and liquid increases

  2. gases increases and liquid decreases

  3. both gases and liquid increases

  4. both gases and liquid decreases


Correct Option: B
Explanation:

The viscosity of liquid is inversely proporional to the square of the temperature, therefore as we increase the temperature, the viscosity of liquids will decrease.


Similarly, the viscosity of gases is linearly proportional to the temperature. hence as the temperature increases, the viscosity will increase

A spaceship entering the earth's atmosphere is likely to catch fire. This is due to:

  1. The surface tension of air

  2. Air Friction

  3. The high temperature of upper atmosphere

  4. The greater portion of oxygen in the atmosphere at greater height.


Correct Option: B
Explanation:

As the body enters Earth's atmosphere, it falls at very high speeds. This high speeds cause a lot of friction with the air, causing the parts to catch fire.
Option B.

When an object is dropped into a fluid, the extent of splash, due to buoyant force of fluid, depends on:

  1. sliding friction

  2. rolling friction

  3. fluid friction

  4. static friction


Correct Option: C
Explanation:

The splash, due to buoyant force of a fluid depends on fluid friction.

If a toy boat in a tan sinks, the level of water will.

  1. Fluctuate

  2. Decrease

  3. Increase

  4. Remain the same


Correct Option: C

Vehicles are streamlined to reduce 

  1. Static friction

  2. Kinetic friction

  3. Fluid friction

  4. Slidingfriction


Correct Option: C
Explanation:

Fishes have streamlined shape. So, to reduce the fluid friction, the vehicles are made streamlined.

A good lubricant should be highly

  1. viscous and velocity

  2. non-viscous volatile

  3. viscous and non-volatile

  4. transparent


Correct Option: A
Explanation:

A good lubricant generally possesses the following characteristics: A high boiling point and low freezing point (in order to stay liquid within a wide range of temperatures) A high viscosity index. Thermal stability.

Two spheres $A$ and $B$ fall through the same viscous fluid. $A$ and $B$ have the same density and $B$ has the larger radius.
Then,

  1. A has the larger terminal velocity

  2. A and B have the same terminal velocity

  3. B has the larger terminal velocity

  4. Insufficient information is given to reach a conclusion.


Correct Option: C
Explanation:

Two spheres $A$ and $B$ fall through the same viscous fluid $A$ and $B$ have  the same density and $B$ has the larger radius the $B$ has the larger terminal velocity.

$\therefore $ Option $C$ is correct.

Ball pen and fountain pen depends respectively on principle of:-

  1. Surface tension and viscosity

  2. Surface tension and gravity

  3. Gravitation and Surface tension

  4. Surface tension and Surface tension


Correct Option: A

More viscous oil is used in summer than in winter in motors due to

  1. rise in temperature in summer, the viscosity of oil increases.

  2. rise in temperature in summer, the viscosity of oil decreases.

  3. surface tension of oil decreases.

  4. surface tension of oil increases.


Correct Option: B
Explanation:

Due to rise in temperature viscosity of oil decreases in summers. So more viscous oils are used for better lubricant protection.

With the increasing molecular mass of a liquid, the viscosity .

  1. decreases

  2. increases

  3. no effect

  4. all wrong


Correct Option: B
Explanation:

With the increasing molecular mass of a liquid, the viscosity increases.
The flow of molecules is inversely proportional to its mass. Hence, liquids with high molecular mass possess greater viscosity.

When cooking oil is heated in a frying pan, the oil moves around in the pan more easily when it is hot. The main reason for this is that with rise in temperature, there is a decrease in

  1. density

  2. surface tension

  3. viscosity

  4. angle of contact


Correct Option: C
Explanation:

That is true. The temperature dependence of liquid viscosity is the phenomenon by which liquid viscosity tends to decrease (ore, alternatively, its fluidity tends to increase) as its temperature increase.

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