Why do fans have three blades?

Now, just try to figure this out.. Why do most fans have only three blades? why could it be so?? Hmm...Any idea??

Is there any aerodynamic law that supports the efficiency of using only three blades in electric fans? Think.....! Are you thinking of any kinda complicated theorems in fluid mechanics? Are you trying to solve this query using Bernoulli's principle?

All I can say is that it's a vain attempt!

Actually an electric fan can have any number of blades depending upon certain factors. But, why is it so that most fans have only three blades? Let's see here.

Firstly, the main purpose of an electric fan is to create air-flow. So, a good fan must create a good air-flow with minimal noise. Usually an electric fan has a shaft which is rotated at a moderate speed by an electric motor. The speed of electric fans is chosen in a moderate level for safety purpose and to reduce the noise produced when a fan is operated. There are blades attached to this shaft which rotate along with the shaft.

The number of blades and their shape may vary depending upon the model and aesthetic sense. Even there are certain fans with only one blade. These are usually meant for aeroplane propellers. (see bottom)

Fans may have any number of blades placed at any position. But the weight of the blades must be balanced so that there is maximum efficiency. For that reason, fans with only one blade is not often preferred, as the weight of the blade tend to bend the blade down.

Coming to the arrangement of fans with two blades, it is obvious that these two blades must be oppositely arranged to each other. Similarly in the case of having three blades, each of them must make an angle of 120 degrees with the next blade. 

Now coming to the structure of the blades, each blade is moderately long, thin and bent a little up. Usually metal/alloy blades are preferred for their rigidity and ability to withstand mechanical stress. But, nowadays, there are plastic(polymeric) blades having a comparable strength to metal blades, are used.

Each blade is inclined a bit so that it forces air ahead of it forward. When an electric fan is switched on, the rotation of the blades increases the velocity of the air along the blades (in circular direction) Generally hot air rises upwards. This hot (less dense) air comes to contact with the whirl(rotation) of the blades and are forced back with a little higher pressure. Thus air-flow is created.

An interesting point to note is that an electric fan never creates a cooling effect, by itself. Try switching on a fan in an insulated room. After some time, the temperature will rise a bit. This is because, the electricity used by fan is not completely converted into mechanical energy, as per the second law of thermodynamics. Some amount of the electric energy is converted into heat energy due to the resistance of coil and several other effects and thus the temperature rises. But every-time when we are laid down under an fan, we feel cool breeze, isn't that true? How it happens?

The cooling effect produced by a fan is due to the fact that the air it pushes down makes our sweat evaporate at a faster rate thereby decreasing our temperature. Thus, we feel chill! So, forgot your tension, sit back and relax!

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Images in the article are taken from the web.

Why can't we push a bus from inside?

Now that we are all outing for a picnic. The bus driver shouts as there's a failure in the engine's start-up. We will do what all we can do! Some will panic while some will make others panic and some will shout at the driver and the rest will look cool undisturbed. The most optimistic approach to this situation is pushing the bus till it starts up. We all know that. While everyone is trying to push the bus, what if one pushes the bus from inside??

It's absurd idiot-ism. But "Why actually we  can't move a bus from inside?" Hmm?? Let us give it a try.

According to Newton's first law of motion, "Every body continues to be in the state of rest or in uniform motion, until an external force acts upon it".

Newton has well described the law by including the term "external" which says that there is no change of position of the object( or its centre of mass) without the application of an external force. But what exactly is this "external force"? Well, any "unbalanced" force acting on the system is said to be an external force. 

In order to get a clear picture, we shall compare the properties of internal and external forces. Generally speaking, any internal force is "balanced", i.e, there are forces whose resultant is zero, whereas there is a finite resultant in case of an external force.

When we are pushing the bus from inside, our hands are providing a finite force on the walls of the bus, while our legs are pushing the floor of the bus with more/less the same force. These forces are normal(perpendicular) to each other.

These forces cancel and the resultant comes out to be zero. Hence we cannot move the bus. 

However if there is no friction and if we run from one end of the bus to the other end, the bus moves in the opposite direction so as the keep the centre of mass in the same place. In the absence of any other force, a system tries to keep the centre of mass in the same place despite the action of a small force.

Try running from the back of the bus and jumping into the front side such that your legs hit the front surface in karate style. As no part of your body is in contact with the bus at the instant when you hit the bus, your force is purely external and moves the bus. 

And that's the same reason "why we can't lift the chair in which we are sitting". 

Can you lift yourself?? Just give it a try.