Gravitational potential energy কী?

Example 9.5 (Water tower)

Each cubic metre ($1000\,\mathrm{kg}$) of water stored $30\,\mathrm{m}$ up a water tower holds $1000 \times 9.81 \times 30 \approx 2.9 \times 10^{5}\,\mathrm{J}$ — about six tenths of the car’s kinetic energy above. Cities stockpile energy overhead.

Example 18.8 (A book on a shelf)

A $1.2\,\mathrm{kg}$ book sits $1.8\,\mathrm{m}$ above the floor, itself $9.0\,\mathrm{m}$ above the street. Reference at the floor: $E_p = 1.2 \times 9.81 \times 1.8 \approx 21\,\mathrm{J}$; in the street: $E_p \approx 127\,\mathrm{J}$. Falling to the floor releases $21\,\mathrm{J}$ in both accounts.