In physics, mass measurement is used to determine the amount of matter in an object. It is often compared to weight, which is a measure of the force of gravity on an object.
Both measurements are made using a numbering system. The most common metric units are the meter, kilogram, and liter. Equal quotients indicate an equal ratio between the two numbers.
Weight
Weight is the force of gravitation acting on an object, determined by its mass and its acceleration due to gravity. Objects are measured for their weight using spring or balance scales, which work by comparing the object with references (either other objects or zero-gravity containers).
Your “weight” will depend on where you measure it — a pineapple has different weight than a wooden baseball bat. This is because weight depends on the strength of gravity, while mass depends only on the amount of matter it contains.
The SI base unit for mass is the kilogram, defined as the mass of the international prototype, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures in Sevres, France. Six prefixes are used to define the smaller units of the metric system. They are shown in the table below.
Density
Density is a fundamental measure of the amount of mass contained within a given volume. It is an important concept for many different science fields, from physics and geology to biology and chemistry. It can help scientists predict how substances will interact under certain conditions, like whether a solid substance will sink or float in a liquid. It is also used to differentiate substances that might appear similar to the human eye, such as comparing gemstones of the same colour.
A material’s density can be determined by dividing its mass by its volume. This can be done using a variety of techniques and equipment, such as a hydrometer (for liquids), an immersed body balance (for solids), or an air comparison pycnometer (for liquids and solids). There is a long tradition of beginning physics with careful measurements of volume and mass and calculations of density. Students may find the calculation tedious, since it requires an attention to detail that is often more than what they are accustomed to at school.
Force
Engineers in disciplines involving weight loading (force on a structure due to gravity), like structural engineering, understand the distinction between mass and force. They use a formula, based on Einstein’s famous equation E = mc2, to convert the object’s mass into its corresponding force.
Those who work with rigging also know that a device’s force calibration is affected by local air density and material densities, not only by its mass and the local gravitational acceleration. This is why a force-measuring device calibrated in one location using mass weights must be checked against a reference standard, such as a standard kilogram of mass and the non-SI kilogram-force or avoirdupois pound of the English system of measurement.
Inaccurate mass and force measurements have serious consequences, from the quality of products and equipment to the safety of people working with them. That’s why the accurate measurement of these quantities is so important to industries including aerospace, construction, automotive, electronics, manufacturing and pharmaceutical.
Gravity
In physics, gravity is one of the most important forces that governs our world. It is the force that attracts all things that have mass toward every other thing with mass. Isaac Newton elevated the phenomenon from inscrutable tendency of objects to fall to the ground to a well-understood and predictable force that holds all matter together.
For example, if you drop two identical clocks at the same time from a tall mountain peak and then switch their positions at sea level, they will run at different rates, reflecting the effect of gravity. Gravity is also responsible for the fact that a balloon rises higher in the atmosphere than it does at the bottom of a deep valley, and for why a car rolls faster down an inclined plane than it does on a flat surface.
But measuring the force of gravity is a challenge. The best way is with a device called a torsion balance, used in labs at NIST and around the world.