The Fast Moving Vehicles Are Given Streamline Shape. Why ?

Hint:A streamlined body is a form that reduces friction drag between a fluid and an item moving through it, such as air or water. Drag is a force that slows motion down; friction drag is a type of drag. When the fluid nearest to the item adheres to its surface, it exerts a force that resists the item's motion.

Complete answer:
Many species, such as birds and dolphins, and technology, like aeroplanes and submarines, have streamlined bodies to decrease friction drag while moving through air or water. A streamlined body is a form that reduces friction drag between a fluid and an item passing through it, such as air or water.

Drag is a force that slows down a moving object. Fast-moving vehicles are given a streamline design because it allows them to cut through the air more readily, increasing their speed. Aeroplanes in the air and ships on the sea have streamlined shapes to decrease fluid friction. At small angles of attack, a streamlined body resembles a fish or an airfoil, but at large angles of attack, a bluff body resembles a brick, a cylinder, or an airfoil.

Frictional drag is the primary source of air resistance for streamlined bodies. Pressure drag is the primary cause of drag for a bluff body. A streamlined body will always have less resistance than a bluff body for the same frontal area and velocity. A streamline is the route that a massless particle follows as it flows with the flow. Because the streamline is traced by a moving particle, the velocity is parallel to the route at all points along the route.

Note: The notion of streamlines is a crucial one in the study of aerodynamics. The forces are the same whether the aeroplane is moving through the air or the air is being pushed past it. Before attempting to fly an aeroplane, it is typically easier, less expensive, and (in certain situations) less risky to test it in a wind tunnel.