Suppose you drop a glass of milk that smashes into 27 pieces and milk spills all over the floor. If you videotaped this event and ran the film backwards it would be obvious to you that the film was running backwards. Why?

The process violates:

1. Conservation of Energy
2. Conservation of Momentum
3. The Second Law of Thermodynamics (Entropy Never Decreases)
4. All of the above

Only the Second Law would be violated - this is why entropy is sometimes called time's arrow. Time advances in the direction of increasing entropy.

The broken glass is an example of an irreversible process, where mechanical energy is converted into heat, or if heat flows from a hot to a cooler region.

A reversible process is one in which there is no change in entropy, and the system and the surroundings can be returned to the initial state.