A "fermi" (symbol: fm) is a unit of length used in physics to describe extremely small distances at the atomic and subatomic scale. One fermi is equal to 10^-15 meters or 0.000000000000001 meters. It is named after the Italian physicist Enrico Fermi, who made significant contributions to nuclear physics and particle physics.
The fermi is particularly useful for describing the sizes of atomic nuclei and the distances between particles within atomic nuclei. For example, the typical diameter of an atomic nucleus is on the order of a few femtometers (1 femtometer = 1 fm). It is also used in high-energy particle physics to describe the cross-sectional area of particle interactions.
In summary, the fermi is an essential unit of length for understanding the microscopic world of atoms, nuclei, and subatomic particles, where distances are incredibly small.
A kilometer (km) is a unit of length used in the metric system of measurement. It is equal to one thousand meters, which is the base unit of length in the International System of Units (SI). Therefore, 1 kilometer is equal to 1,000 meters, 100,000 centimeters, 1,000,000 millimeters, or 0.621371 miles.
Kilometers are commonly used for measuring longer distances on Earth, such as the length of roads, the distance between cities, and the dimensions of geographical features like rivers, mountains, and coastlines. They are also used in various sports and outdoor activities, such as running, cycling, and hiking, to express distances. Kilometers are a convenient unit of measurement for everyday use in many countries around the world that use the metric system.