Many people are confused about the terms mass and weight. Mass is a property of matter, and weight is a measure of the gravitational force on an object.
Weighing instruments (such as balances) use a concept called relative mass to determine the object’s weight. Thus, a person’s weight on the Earth would be different than on the Moon or in space.
Units
The SI unit for mass is the kilogram, represented by kg. As with the other 7 base units, the kilogram can be combined to form derived physical quantities such as speed and acceleration.
The unit of mass is not the same as weight, which measures the force exerted on an object by gravity. An object on the Moon, for example, would weigh less than an identical object on Earth due to lower gravitational pull.
Scientists are currently evaluating alternatives to the current definition of the kilogram, which has been fixed in terms of fundamental constants of nature since 2019. One such alternative is a new definition of the kilogram based on the dalton, or unified atomic mass unit (u), defined as the amount of carbon-12 atoms in their ground state at rest. This would allow a kilogram to be redeemed at any time from the International Prototype Kilogram, a platinum-iridium cylinder kept by NIST. It would also be compatible with a proposal to fix the Planck constant h in terms of the atomic masses of hydrogen and carbon.
Measurement Methods
Whether we are buying food, taking medication, designing a bridge or space shuttle, or trading grain or gold, mass measurement impacts our lives in many ways. We depend on accurate, consistent standards to perform these measurements.
The kilogram (kg) and gram are the SI units for mass. These are the standardized units used by scientists around the world, allowing for consistency in measurement data.
Very small masses and forces have historically been measured with sets of carefully calibrated metal weights. This method has several drawbacks, however. First, the weights can erode over time and cause inconsistent results. Second, the smallest weights available are only about 1 milligram, making them insufficient for some applications.
NIST’s newest mass measurement instrument provides an optomechanical reference, a key building block for cheap field-portable balances and other devices that would enable mass measurements on the nanoscale. These new devices use a dual Single-Measurement-of-Buoyant-Mass Refiner (SMR) system. Each SMR operates in a different fluid, and the two instruments are connected at a cross-junction. In this configuration, multiplicative and additive errors are minimized at the point where Fluid 1 density is slightly higher than Fluid 2 density.
Conversions
In order to solve problems in science, it is important to be able to convert between different measurement units. This is especially true when working with metric and English system measurements.
For example, in the metric system the unit for mass is kilogram (kg), while in the customary system weight is measured in ounces and pounds. The ability to convert between these units is essential in a world where many countries now use the metric system.
Conversions are accomplished by using conversion factors. These conversion factors are based on equality between the starting and desired units. For example, a conversion factor can be used to change a measurement of distance from meters to kilometers by multiplying the distance by 1,000. The same method can also be used to convert between metric and English system measurement units for length, area, and capacity. It is important to keep track of which unit cancels out in the final answer and which units remain, as this indicates whether the chosen conversion factor is correct or not.
Applications
In everyday life, we most often measure mass using a scale (balance or weighing machine). These machines balance an unknown object against known weights to provide a readout of the object’s mass. This type of measurement takes g into account, since the force of gravity is proportional to an object’s mass.
We also use sophisticated instruments to measure an object’s mass, such as triple beam balances and analytical balances. These instruments measure very small masses to the nearest microgram or milligram with extreme precision.
The BIPM maintains facilities for hydrostatic and immersed density measurements to support research on alternative materials for the platinum-iridium prototype kilogram (PIK) and other mass standards as well as for characterization of the magnetic properties of Pt-Ir metals [1]. Moreover, it conducts periodic verifications of the IPK and its national prototypes to demonstrate that the unit of kilogram is accurate over an appropriate range. This is a crucial step towards a possible redefinition of the kilogram based on fundamental principles.