Fluid dynamics - class-XI

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.

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.

Viscosity of air is responsible for floating of clouds is air

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.

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 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.

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 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. 

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.

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

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

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.

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.

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

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

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 the $B$ has the larger terminal velocity.

Ball pens use a ball that rolls ink onto paper, relying on surface tension to keep the ink in the reservoir. Fountain pens rely on capillary action and gravity to feed ink to the nib.

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

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.