Question

why does a fan keep moving for some time after switched off

Answer 1

Newton’s First Law of Motion. It states that a rigid body at rest stays at rest and a rigid body in motion stays in motion with the same speed and same direction, unless and until an external force is being applied to the rigid body. And this law is also referred to as Law of Inertia.

Now here according to your question, the fan here is the rigid body and the rigid body is in motion when the switch is turned on. When the fan is switched off, here rotational inertia acts upon the rotating fan. And thus due to this rotational inertia the fan rotates,i.e., its blades moves,and will rotate until and unless an external force is being applied. And since there is a frictional force acting upon the rotating blades of the fan due to the air, the speed of the fan blades ceases and ultimately comes to halt. Thus here, air friction is the external force which makes the fan blades to stop after sometime, otherwise it would have never stopped rotating

Answer 2

The fan blades keep on rotating for some time after switching off, because of inertia. More specifically, it is inertia of motion, defined by Newton's first law of motion.

Now, to understand how long the blades will continue to rotate, we need to look at the kinetic energy stored in the fan (with blades), when it was being driven by electric power. Any moving or rotating object possesses kinetic energy due to its motion. Because of this stored energy, the blades will continue to rotate and work will be done by the blades in a) moving air from top of the fan to bottom (in case of a ceiling fan), though in lesser volume and b) overcoming the frictional resistance acting on the blades due to its movement through air. Thus the kinetic energy will get consumed by doing work and when stored energy is fully exhausted, the blades will come to rest.

^=^