Theory:

What is fluid?
A substance that moves easily and changes its shape, such as liquids and gases, is called fluids.
Even though the air is very light and thin, it exerts a frictional force on objects moving through it. Similarly, liquids like water exert the force of friction when objects move through them. In science, fluids are the common name of gases and liquids. We can say that fluids exert a force of friction on objects when it is moving through them.

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Air friction that acts on the parachute makes it move slowly.
 
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Water exerts fluid friction on the moving boat.
The frictional force exerted by fluids is also called drag.
  • The frictional force that acts on an object in a fluid depends on its speed with respect to the fluid.
  • The frictional force depends on the shape of the object and the nature of the fluid.
  • To move the objects through fluids, it has to overcome friction acting on them.
  • When a object moves in the fluids, they lose energy due to friction. Therefore, efforts are made to minimize friction by giving extraordinary shapes to the objects.
  • Birds and fish must constantly move around in liquids. Their bodies had most likely evolved into shapes that would allow them to defeat friction without wasting a lot of energy.
  
Compare the similarity in shape of a bird to the aeroplane
 
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Shape of a bird and an aeroplane