A special class of Commonsense

 I intend to throw light on a certain category of commonsense which, as said just before, is commonsense, but follows a special trend. These pieces of commonsense are easy to agree with, or realize/understand to be correct, after being told about them, but hard to be struck by during the course of being in the situation (or problem-solving situation).

 

Here are a few examples –

 

1)      All that is written in the world – be it magazines, newspapers, books, phones, internet, computers etc. – is written BY A HUMAN BEING. So, all written data is man-made. (Nothing has descended from the sky or nothing that is written in a certain script has been created by a process of nature).

This is commonsense – all written data everywhere in the world is man-made - but not something everyone has realized.

 

2)      Sometimes the key thing to realize in a problem is the “smart” commonsense point, and not, say, the laws of the subject (say, Physics).

Consider this problem- There is a solid disc. It is rotated along a fixed axis which is in the plane of the disc but doesn’t pass through the centre (is not its diameter). There is an ant on the circumference-edge of the disc at the farthest point from the axis on the circumference of the disc, in the beginning and it is moving along the surface of the disc in a straight line. Find the velocity of the ant when it reaches the other end of the disc.

Here, you write the momentum conservation equations of the system (linear and angular). You write the energy conservation equation (potential, kinetic, rotational kinetic energies). And then you fall one equation short considering the number of variables.

The key here is to realize that the angular velocity of the ant at both the end-points is the same! If the axis of rotation was the diameter then this is easy to realize. But the axis being a chord of the disc, it is quite elusive to realize that the angular velocities are the same at the 2 extremes of the disc (initial and final positions of the ant) (even though the linear velocities aren’t (‘r’ X ‘angular velocity’)). The distances of the 2 points from the axis are different making the linear velocities different, but the angular velocities are the same. This is because the angle by which the 2 sectors of the disc rotate in a given amount of time, for any angle of rotation, are the same. They are vertically opposite angles.

Now, this is not a Physics point, it’s a commonsense point. But in the situation of solving the problem (with the ant, the variables of linear and angular velocity, conservation equations etc.) becomes obscure.

 

3)      Consider a graffiti board. It is full. Now, the stuff written near the borders is the recently written stuff. 

This is because people will start writing in the free middle-spaces when the board is blank and move towards the edges once space starts getting used up! The statement in bold, is commonsense but again, quite elusive to realize on seeing a fully-occupied graffiti board.