MASS
Mass or Weight?
It is important to distinguish mass from weight.
Weight is defined as the gravitational force (generally that of the Earth) acting on a mass. The SI unit of weight, according to the International Bureau of Weights and Measures, is the Newton (N).
Mass, on the other hand, is a constant property of a body, independent of its environment. Its official unit of measurement is the kilogram.
For a clearer understanding, let us imagine a volume of 1 litre of water.
Its mass, measured with a balance, will be 1 kilogram (compared to a 1 kg reference weight, the balance will always remain in equilibrium).
The weight of this same litre of water on Earth, measured with a dynamometer, is expressed in Newtons (N). Weight is the product of mass × gravitational acceleration. Thus, the mass of 1 kg for our litre of water, multiplied by the Earth’s average (and very constant) gravitational acceleration of 9.81 m/s², gives a terrestrial weight of 9.81 N.
Now let us take our litre of water to the Moon.
If we measure its mass with a balance, it will remain in equilibrium against a 1 kg reference weight. However, if we measure its weight with a dynamometer, it will be drastically reduced. In this case, 1 kg × 1.62 m/s² (lunar gravity) gives a weight of 1.62 N. In other words, a lunar weight six times lower than its terrestrial weight, while its mass remains unchanged.
Since the Earth’s gravitational force is both highly constant and universally accepted, weight is commonly (though incorrectly) expressed in kilograms, which explains the frequent confusion between these two values.
A decisive factor in the choice of materials
In watchmaking, as in many mechanical applications, the mass of components is a decisive factor in material selection. By function and by its asymmetrical geometry, an anchor must be as light as possible. Conversely, balance wheels are valued for their high inertia.
Inertia is the tendency of a body to maintain its velocity despite external factors (gravitational forces, shocks, temperature variations, etc.). The inertia of a body depends directly on its mass.
As a determining factor in a watch’s chronometry, the mass of movement components is the subject of the utmost attention and continual research.