A mass falls to the ground from a height. What's the change of the entropy of the universe? A mass $m$ falls to the ground from a height $h$. The temperature $T$ is constant. What's the change of the entropy of the universe?
It's an example in Carter's Classical and Statistical Thermodynamics. The book gives this answer:
The process is irreversible but we can imagine the mass being slowly and reversibly lowered by a string and pulley arrangement. Since no heat exchange is involved, $\Delta S_\text{system} = 0$, but $$\Delta S_\text{universe} = \Delta S_\text{surroundings} = \frac{W_r}{T} = \frac{mgh}{T}.$$ We note that for the surroundings, which are unchanged if the mass is small, $$T\Delta S=Q_r=\Delta U+W_r=0+mgh.$$
My question is about the surroundings. They do work on the system so $W_r=mgh$ seems plausible. But according to the first law of thermodynamics, the surroundings also absorb heat $Q_r=0+mgh=mgh$. I wonder where the heat comes from. Is it generated after $m$ falls to the ground and becomes stationary?
If this is so, then another question arises. Before $m$ touching the ground, what gives the surroundings heat to let them do work on $m$?
 A: The potential energy you list, $mgh$ is converted into kinetic energy. When the mass $m$ hits the ground, its kinetic energy is transferred into vibrations in the surroundings which become disorganized over time, eventually becoming random motions of small particles which we call heat or thermal energy. So, gravitational energy is ultimately converted to heat in this process, and this heat spreads through the surroundings. At the end of the day, there is heat transfer.
On the other hand, if you catch the mass with a spring and then stop the spring from rebounding, the gravitational potential energy is converted to spring potential energy, not to heat. If you released the spring, the mass would return to its initial height before falling again. Since this is perfectly reversible, there is no entropy change.
A: Here. It is the case of entropy generation due to mechanical irreversibility.as change in entropy=dq/T + entropy generated
In irreversible cases.There is no heat interaction between body and surroundings.
