Hide and seek

The Joke

All the scientists are in heaven and they are full of boredom. Then Lagrange has the idea of playing "hide and seek".
- "Yeaaaah", everybody cheers and they decide that Einstein should count.

Einstein start counting and everybody runs off and hides. Heisenberg tries tunneling effects to hide behind a wall, Chandrasekhar creates back holes to jump in, Feynman integrates himself over all possible paths-hides, Dirac acts with a descendant operator on himself and so on.

Only Newton doesn't move but draws a square of side 1m and stands right behind Einstein.

After Einstein finishes counting, he opens his eyes and all surprise finds Newton first
- Hey, found you Newton! That was too easy, it's your turn to count.
Then Newton says quite aggresively (he was already upset with Einstein because of his new theory of gravity...you know...)
- No you didn't!!! You found Pascal, not me.

Everybody is astonished, how can this be possible? Einstein challenges Newton to prove his claim. All the others come out of their hides and sit down on a circle to listen to the well-known geometrical proofs of Newton. Then he says

- Well gentlemen, I am Newton in a square meter, pointing down.
- Therefore I am a Newton per square meter, thus I am Pascal! QED.


Background

Pascal is the unit of pressure in the International System of units (SI). But pressure is defined as force per area, which means how much force is applied on a square metre, i.e.

$P = \frac{F}{A}$

Since there is a relationship between different physical quantities, their units must respect this relationship too. So we only need to recall that the unit of the force in Si is [Newton] and the unit of area is [square metres], i.e.

$[Pascal] = \frac{[Newton]}{[m^2]}$.

In other words Pascal is "a Newton in a square metre".

People very often confuse force with pressure and tend to think of these two different physical quantities as the same. To distinguish between them think of this easy example; imagine you take a pin and apply it with some force on your skin (please don't try it, just imagine it). Then take an one pound coin and apply it to you skin with the same force. In the first case the pain was much more intense. What actually determined the size of your pain was the pressure your skin felt, not the force. The pin has very little area (just a tiny spike) and therefore the pressure is much bigger than the pressure you felt from the coin (it has a huge area compared to the spike of the pin).

For more details follow http://en.wikipedia.org/wiki/Pascal_(unit).