Mass and its measurement

A toy car loaded with sand is easy to set moving, a real lorry is hard; a rolling toy car is easy to stop. So bodies tend to keep both their state of rest and their state of motion. The ability of a body to keep its state of rest or motion is called inertia; it differs from body to body. The physical quantity describing inertia is the mass of a body (symbol m). The SI unit of mass is the kilogram (kg): 1 t = 1000 kg, 1 kg = 1000 g, 1 g = 1000 mg; 1 centner = 100 kg. For a long time the kilogram was defined by a platinum-iridium cylinder kept at Sèvres near Paris; since 2019 it has been defined through a natural constant (the Planck constant). Mass is measured with a balance. A beam balance is the most reliable: it does not depend on where the body is or on its temperature, so in calculations mass is taken to be constant (m = const). A spring balance depends on the state of the spring and on location, so it is less accurate.