• Fluid Friction

    When a fluid is in contact with an object, the force resisting motion of the object through it is called fluid friction. Thus, fluid friction is the opposition force experienced by the object moving through the fluid. It acts between the object and the fluid through which it is moving. It is also called drag.

    Air is very light and thin, yet it exerts a frictional force on objects moving through it. When an object moves through the air, it pushes the air out of the way and air pushes back on the object. This push of air on the moving objects creates friction which tends to slow down the moving object. Air exerts a frictional force on cars, buses, aeroplanes, rockets and birds moving through it. Air drag, the frictional force exerted by the air on a body moving through it, decreases automotive fuel economy but makes parachutes work.

    Similarly, water exerts a force of friction on the object which moves through them and opposite their motion. When an object moves through water, it pushes the water out of the way and the water pushes back on the object. This push of water on the moving objects creates friction which tends to slow down the moving object. Water exerts a frictional force on objects like boats, speed boats, ships, submarines and fishes which move through it.