MASS
It is important to differentiate between mass and weight.
Weight is characterised by 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 one litre of water. Its mass, measured using a scale, is one kilogram (compared to one kilogram of standard weight, the balance will always be in equilibrium).
The weight of this same litre of water on earth is 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 one kilogram of our litre of water multiplied by the average (and very constant) gravitational force of the earth (9.81 m/s²) provides an earthly weight for 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 scale, it will still be in equilibrium with one kilogram of weight.
However, if we measure its weight using a dynamometer, it will be drastically reduced. Thus, one kg × 1.62 m/s² (lunar gravity) is 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 is usually (and incorrectly) also expressed in kilograms, hence the legitimate confusion between these two values.
Determining criteria for choice of materials
In watchmaking, as in many mechanical applications, an object’s mass determines the choice of materials. For example, a pallet fork must be as light as possible due to its function and asymmetrical geometry. However, the balance wheel with its high inertia must be assessed. 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.
A crucial factor in a watch’s chronometry, the mass of its movement’s components is a subject demanding great attention and constant research.
