Mass

Mass The SI unit of mass, the kilogram (kg) was once defined as the mass of one liter of water at 4°C. (A volume of one liter is equal to the volume of a cube 10 cm on an edge.) Like the standards for time and length, the kilogram standard has changed over time. The kilogram is now defined to be the mass of a specific platinum-iridium alloy cylinder. This cylinder, called the standard body, is kept at the International Bureau of Weights and Measures in Sèvres, France. A duplicate of the standard body is kept at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. We shall discuss the concept of mass in detail in Chapter 4, where we will see that the weight of an object at a given location is proportional to its mass. Thus, by comparing the weights of different objects of ordinary size with the weight of the standard body, the masses of the objects can be compared with each other.