The kilogram (kg) is the base unit of mass in the International System of Units (SI). It is defined as the mass of the International Prototype of the Kilogram (IPK), a platinum-iridium cylinder kept at the International Bureau of Weights and Measures (BIPM) in France.
Until May 20, 2019, the kilogram was defined by the physical prototype, but as part of the redefinition of the SI units, the kilogram was redefined in terms of fundamental constants. Since that date, the kilogram is defined by fixing the numerical value of the Planck constant (h) to be exactly 6.62607015 x 10^-34 joule-seconds.
This redefinition ensures that the value of the kilogram is now based on a constant of nature rather than relying on a physical object that could potentially change over time. It allows for a more precise and stable definition of the kilogram and other SI units.
A milligram (mg) is a unit of mass in the metric system and the International System of Units (SI). It is equal to one one-thousandth of a gram, which means that 1 milligram is equivalent to 0.001 grams. The milligram is a small unit of mass commonly used for measuring very small quantities of substances, especially in scientific, medical, and pharmaceutical contexts.
For example, when you see the dosage of a medication listed on a prescription or on over-the-counter drugs, it is often measured in milligrams. Additionally, in laboratory settings, chemicals and substances are frequently measured in milligrams due to its precision in measuring small quantities.