Lets talk a bit about fluid. The more important fluid that we need to focus on is air. However we can exercise our understanding by using a different fluid for different situation. We want to understand Drag in aircraft. This doesnt seem easy to understand when we want to visualize it as the aerodynamic drag that we face in our daily life is rare. However lets talk about water as the fluid. As we go sometimes swimming or just when we wash our hands in a stream of water, we can feel the water pushing us. This is a force exerted by a fluid on a body. Just the reverse of this would be when we move our hand or body in water. Lets take an easy example of swimming pool . If we try to move within the swimming pool filled with water we feel a sense of resistance. as we try to move. what we are basically doing is moving the molecule of water and allowing that motion to take place. Similar phenomenon happens in aircraft when it speeds up in the run way to take off and when it fly in the cruises