Objects have mass and kinetic energy. The more mass an object has, the greater its kinetic energy.

Everything you see, touch or hold has mass. Mass is a measure of an object’s inertia or resistance to change in its speed or position when force is applied.

People often confuse mass with weight, however, these are two different physical properties. An object’s weight changes due to gravitational pull, but its mass remains the same.

## Units of Measurement

There are several units of measurement used to determine mass. The most common metric unit is the kilogram. The kilogram is a fundamental part of the SI (International System of Units) and is defined using a physical prototype. Other basic SI units include the second, meter (length), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity).

Each of these basic SI units may be multiplied or divided to create larger or smaller units. For example, the liter is equal to one-tenth of a meter cubed, which is 1,000 centimeters squared or 1,000,000 cm3. When converting from other measurement systems, it is important to understand that the different types of measurement units are not interchangeable. For example, the metric unit of volume is liter, while the imperial unit of volume is cubic foot. These units are not based on the same physical constants. This is why it is so important to use a coherent system of measurement units.

## Calculating Mass

Mass is the amount of matter contained in an object. It is a property that does not change, unlike velocity or acceleration, which can be influenced by energy.

You can measure an object’s mass by putting it on a balance or scale and placing objects of known mass on the other side of the scale so they cancel each other out, like on the picture to the right. You can also use a more sophisticated weighing machine that has strain gauge or frequency shift technology load cells for greater accuracy.

It is important to understand the difference between mass and weight. Many people use the terms interchangeably, but they are different physical properties. The term “weight” describes the force exerted by gravity, while the term “mass” refers to the total amount of matter an object contains. An object’s heaviness can vary from place to place, depending on its distance from Earth and its gravitational field strength.

## Measuring Density

The ability to measure density is a skill that helps students unravel fundamental characteristics of substances, objects and even larger scale phenomena like planetary bodies or stars. However, density is only an indirect measurement and it depends on accurate measurements of two other quantities – mass and volume.

In the classroom, teachers often use a set of Tillich bricks or a box of sugar cubes to introduce students to this concept. Then, students learn to calculate density by comparing the weight of an object with its volume (see diagram).

Digital density meters measure liquid samples directly, offering precise, accurate results in a short period of time. They rely on the principle outlined by Archimedes that a body immersed in water will gain or lose an apparent weight equal to the weight of the fluid it displaces. The measurement is traceable to a specific reference standard such as pure water or to a specific concentration of a liquid solution (e.g., ethanol).

## Measuring Weight

Although the term weight is often used to refer to the force exerted by gravity, it actually describes an object’s mass (SI unit = kilogram, kg). An object’s weight is the result of its mass times standard gravitational acceleration. The General Conference on Weights and Measures defines standard gravitational acceleration to be 9.80665 m/s2.

An object’s weight will vary depending on its location. An object’s weight will be less on Mars, where the strength of gravity is much lower; more on Saturn, where the gravitational field is stronger; and very small in space, where there is no significant gravitational effect.

The most accurate way to measure weight is with a balance or spring scale calibrated using stainless steel standards. When measuring a person’s weight, it is important to have them stand on the scales with their feet together and with their hands hanging loosely at their sides. Ideally, the measurement should be repeated to ensure consistency.