Until the time of Newton, what we now know as mass was called “weight.” You can measure the weight of objects on a balance scale.

Your weight may change if you are curled up in bed, or sitting up straight, but your mass stays the same. That’s because passive gravitational mass does not change.

## Metric System

The modern metric system is used by almost every country in the world. It dates back to a French vicar named Gabriel Mouton who conceived the idea for it in 1670. He suggested that the base units of length, time and mass be based on immutable natural properties. All other metric measurements are derived from these base units. For example, force is a combination of the meter, kilogram and second, while volume and area are derived from length and mass.

Year 4 students often learn how to use equivalences (multiplication and division) to convert between different metric measurement units, such as millimetres to inches or kilograms to pounds. The metric system is also around us in everyday life. For example, paper sizes are all metric. Jesse can work out how much postage a package will need by measuring its size in metric units.

The most basic metric unit is the meter, which is just over three feet long. A standard sized piece of paper weighs about one gram.

## Weight

Over time, there have been a huge number of units invented for measuring weight. The baseline measurement unit for weight, however, is the newton. This is because weight is the amount of matter an object has multiplied by gravity’s acceleration.

Mass and weight are not the same things, but they do have a direct relationship. While mass measures the amount of matter an object has, weight depends on its location and varies depending on the strength of the gravitational pull.

Measurements of an object’s mass can be made using balances or weighing scales. The SI unit for mass is the kilogram, or g. Other units of mass include the avoirdupois pound, or lb, and the Newton (Newton), the basic measurement unit for force. The metric system also uses kilograms, but its weight measurement units differ from those of the old-fashioned imperial systems. This difference is accounted for during the manufacture of precision mass standards, and the result is that an object with the same label will appear to weigh slightly different on different balances or scales.

## Inertia

The moment of inertia (MOI) is a key parameter for the dynamic modeling of mechanical systems. It is derived from an object’s mass and shape through calculations or obtained experimentally using a bifilar vertical-axis torsional pendulum.

Inertia refers to an object’s resistance to a change in its motion, such as a change of direction. This property is described by Newton’s First Law, which states that an object at rest or moving at a constant speed will continue to do so unless acted upon by an unbalanced force.

You can measure the inertia of a selected geometry, assembly or part using the Measure Inertia dialog box. The results are displayed in the specification tree and a dialog box. A Keep Measure option lets you keep inertia measures as features and a Customize… option allows you to specify what will be calculated and exported to the text file. Inertia measurements are not associative when switching between folded and unfolded view (using Fold/Unfold in the Sheet Metal toolbar). Only core material is taken into account for inertia calculation, not covering material.

## Gravity

Gravity is one of the four fundamental forces that govern matter. It is what holds the planets in their orbits and the oceans in their waves. It is what keeps stars in their shapes and holds atoms together and even stops light from escaping black holes.

Mass measurements are usually performed with a balance, also known as a lab scale or analytical balance. This instrument compensates for the acceleration caused by gravity by counteracting it with an equal downward force on another pan that is known to have the same mass.

Several factors can influence the measurement of G, including density variations, metrological limitations and magnetic properties of the source masses used for a particular experiment. These issues are typically discussed in the experimental arrangements and error budget sections of each individual published result for a given G determination. In particular, the use of spheres versus cylinders as the attracting masses may lead to differences in G due to the size, shape and properties of these sources.