# Where did the droplet’s energy go? (The second law of thermodynamics and irreversible phenomena)

* In this simulation, the attraction between particles is not considered.

## Where did the droplet’s energy go?

Let’s think about the free fall of a water drop. From a macro perspective, falling water droplets change position over time. In this case, we say that the water droplet moves. And, we can calculate its kinetic energy ($$=\frac{1}{2} m\upsilon^{2}$$).
When this water drop hits the floor, the motion of the water molecules changes completely randomly. Now that the water droplet is still on the floor, the kinetic energy becomes ‘0’. Now, a droplet’s energy can be calculated only with the particles’ average kinetic energy (a value proportional to the absolute temperature). The kinetic energy of individual particles can only be treated as a statistical distribution.
Eventually, the kinetic energy of the water droplet is converted into heat energy.

## Irreversible phenomena

A phenomenon that can return to its original state, such as electrons moving in a vacuum, is called a ‘reversible phenomenon.’
However, most natural phenomena are ‘irreversible phenomena’ that occur only in one direction.

Let’s take an example of the water droplet above. Can the water droplets on the floor pop out using their own heat energy?
When many water molecules move in one direction, water droplets can pop out. However, this doesn’t happen because the water molecules each have disordered movements. (The probability is almost ‘0’.)
As such, most natural phenomena are irreversible, occurring only in one direction.