In our everyday lives, we encounter the metric system all the time. Our scales are metric and most items in stores have their price on them in kilograms.

Scientists, however, make a distinction between mass and weight. Mass is an intrinsic property of an object, while weight depends on the force of gravity on it.

Definition

Mass (symbolized m) is a measure of the amount of matter in an object. The unit for mass is the kilogram, one of the seven International System of Units (SI) base units.

An object’s mass determines its inertial force, or resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other objects.

The size of an object does not determine its mass; a large golf ball has the same mass as a small ping-pong ball. An object’s shape also does not determine its mass; a circle has the same mass as an oval.

It is important for students to understand the difference between mass and weight. For example, your 88-pound weight on Earth would change to about 15 pounds on the Moon and even less on Jupiter or the International Space Station. For this reason, engineers who work in disciplines that use gravity-loading calculations convert the mass of objects to a force, usually in newtons, using a conversion factor.

Methods

The most common method for measuring mass is by using a balance. This type of scale measures the force of gravity on an object by comparing its displacement to another object of known mass that is already in place on the scale. This method is especially useful in the lab because it allows you to measure the mass of a solid or liquid using only one instrument.

Other methods for measuring mass use different types of measurements. For example, a massometer, which is used to determine the inertial mass of astronauts while in weightlessness, uses oscillations measured by a force sensor and a dynamics model.

This laboratory provides a chance to investigate how precision affects data in three ways: geometrically (using lengths); by water displacement; and by pycnometry. Students will also calculate the volume of a mixed brass-aluminum cylinder to find its density and learn about the relationship between density and mass. A comparison of these methods will be made with emphasis on the impact of error.

Units

The units for mass are gram (g), kilogram (kg) and tonne (t). The kilogram is one of the seven base units of the International System of Units. It is defined by the mass of a physical prototype such as the platinum-iridium international prototype kilogram, kept at the BIPM in France.

The other SI derived units for mass are joule, newton, and dalton (Da). A metric ton is equal to 1000 kilograms.

A regular-sized paperclip has a mass of about 11 grams. To help kids understand these different units, they can compare it to other objects, like a pen that has about ten times the mass of the paper clip. In countries that follow the metric system, materials are often specified by their mass rather than their weight. However, the term weight is still widely used in the U.S. customary system, where it is typically measured in ounces, pounds and tons. This is an important point for students to keep in mind.

Applications

A common confusion is the difference between weight and mass. Weight depends on gravity, while mass represents the amount of matter in an object. For example, an object may have different weights when it is located on different planets but its mass will remain the same.

For the determination of an unknown chemical compound’s elemental composition, exact mass spectrometry is generally the method of choice. This type of mass spectrometry is capable of providing the unique elemental composition for every ion present in a sample.

However, a balance or scale can also be used to determine an object’s mass. By subtracting the weight of the container and sample, an accurate measurement of the mass can be made. Many science laboratories use this method to measure the mass of liquids or other samples that are too small for a mass spectrometer. Scientists who work in disciplines requiring the calculation of force loads (which include civil and aerospace engineering) will also use this technique.