The base elements → Measurement units

MASS

It is important to differentiate between mass and weight. Weight is characterised by the gravitational force (generally that of the Earth) acting on a mass-bearing body. The unit of weight, according to the International System of Units (SI), is the Newton (N).

The mass of a body, on the other hand, is constant regardless of its environment. Its official unit of measurement is the kilogram.

For better understanding, let’s imagine a volume of 1 litre of water. Its mass, measured using a balance, will be 1 kilogram (compared to a 1 kg standard weight, the balance will always be in equilibrium).

The weight of this same litre of water on Earth will be measured using a dynamometer and expressed in Newtons (N). Weight is the result of the product of mass and gravitational force.

Thus, the mass of 1 kg of our litre of water multiplied by the average (and very constant) gravitational force of the Earth (9.81 m/s²) gives an earthly weight of our litre of water of 9.81 N.

Now let’s take our litre of water to the Moon. If we measure its mass using a balance, it will still be in equilibrium with a 1 kg weight.

However, if we measure its weight using a dynamometer, it will be drastically reduced. Thus, 1 kg × 1.62 m/s² (lunar gravity) gives a weight of 1.62 N. This is a lunar weight six times less than its earthly weight, while its mass remains unchanged.

The Earth’s gravitational force being very constant and generalised is usually (and incorrectly) also expressed in kilograms, hence the legitimate confusion between these two values.

In watchmaking, as in many mechanical applications, the mass of objects determines the choice of materials. For example, a lever must be as light as possible due to its function and asymmetrical geometry. However, the balance wheel with high inertia is appreciated. Inertia is the tendency of a body to maintain its velocity despite external factors (gravitational forces, shocks, temperatures, etc.). The inertia of a body directly depends on its mass.

As a crucial factor in the chronometry of a watch, the mass of the components of its movement is the subject of the greatest attention and constant research.