Fluid friction - class-XII

Friction is the force that opposes motion between any surfaces that are in contact. There are four types of friction: static, sliding, rolling, and fluid friction. Static, sliding, and rolling friction occur between solid surfaces. Fluid friction occurs in liquids and gases.

The maximum limit of static friction is called limiting friction . So,the coefficient of static friction is usually more than that of sliding or kinetic friction. ... Static friction is greater because it works when two surfaces in contact are just to start a relative motion or are in rest.

Lift is the upward acting force which is against gravity. Drag is the resistance force or friction force in fluid. The rolling friction is a resistive friction when a body rolls on a surface. The dynamic friction is the friction that must be overcome to maintain steady motion of one object relative to another because they remain in contact.

Boats and aeroplanes are streamlined so as to reduce fluid friction.
Aeroplanes, boats, fishes and birds which move through fluids have bodies of special shape, called streamlined shape, so as to reduce the friction due to fluid and avoid energy loss. 
For example,if the boat isn't streamlined, then it also has to push the water out of its way, increasing the drag force.

Liquids and gases are together referred to as fluids and the frictional force exerted by them on various objects is called fluid friction.

The speed of the object moving through a fluid affects the amount of friction acting against the motion of the object. More the speed, more is the friction. If the object is broad, it gives more area for the friction to act on. Thus, a streamlined body structure helps to cut through the fluid and reduce friction. Also, the nature of the fluid affects friction.

It is very important to minimize fluid friction for aeroplanes because more friction for aeroplanes may produce more heat, which may harm the plane. Thus, various measures are taken to reduce friction. An optimum speed is maintained and the body structure of the plane is more streamlined so that it can cut through air with minimum friction. Also, such metals are chosen which are light but strong and also make for smooth surface.

A car with streamline body suffers less air friction while moving. And the car with flat face will suffer higher air friction. As a result, the car with streamline body will require less force from the engine to maintain the same speed.

When the arrow moves toward target, the force acting on it, is due to gravity and resistive force of air. The resistance force of air is also called as frictional force.  

With windows open, air comes in and resists the motion of the car. So, due to air resistance, speed of the car reduces.

By definition, fluid friction is a force that resists the movement of a solid object through a fluid.

The other name for fluid friction is drag. It opposes the relative motion of any object in liquids or gases.

This example relates to fluid friction. Speed of spreading of a volume of liquid depends on the liquid friction.Friction offered by water is less as its density is less than oil.

Dynamic Fluid Friction is lower than sliding friction of solids.

Fluid friction increases with the depth of a flowing liquid.

Lubricating machine parts indicates advantage of fluid friction. This helps in lowering the friction and thus helps to keep parts of machinery in good condition for some time. 

Water, having less density than honey, offers lower fluid friction as compared to honey.

Hailstones falling from atmosphere is related to Fluid friction. Fluid friction occurs when there is resistance between particles in fluids or in gases.

Fluid

Fluid friction occurs when a solid object travels through a liquid or gas. Aeroplanes are streamlined to reduce the fluid friction.

A pointed tip is more sharper than a blunt tip. So the air resistance for the pointed tip is less than that of the blunt tip. Since the air resistance opposes the motion, so the pointed can tip move faster than that of blunt tip. Thus, the given statement is true.

Static friction is the force that is the greatest. This is because it opposes motion the strongest between the various types of friction. Friction is greatest in the absent of lubrication,and since there is presence of fluid lubricant in fluid friction it has less friction as compared to solid friction,and in solid friction there is no presence of any lubricant.Hence the given statement is false.