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

## Where did the pendulum’s energy go?

A pendulum that vibrates in the air continues to collide with air molecules in the process of vibrating. Air molecules get their energy from the pendulum. In this process, Air molecules increase their speed. Conversely, the pendulum has lost its energy.
Eventually, the mechanical energy in the pendulum is converted into the air molecule’s heat energy.

Mechanical energy of the pendulum → heat energy of air molecules

## 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 a pendulum. Can the pendulum move itself using the heat energy of air molecules?
If many air molecules collide with the pendulum in one direction, the pendulum can move by itself. However, this doesn’t happen because each air molecule has a disorderly movement.
As such, most natural phenomena are irreversible, occurring only in one direction.