Mass in Physics
Mass is one of the fundamental quantities in physics, representing the amount of matter in an object.
Traditionally, mass was defined operationally as the quantity measured using a balance scale with equal arms, which allows for a direct comparison between two masses by balancing them.
In the International System of Units (SI), the unit of mass is the kilogram (kg).
The Kilogram Redefined: From Physical Objects to Planck’s Constant
Before 2019, the kilogram was defined by a physical reference: a specific cylinder made from a platinum-iridium alloy, kept at the International Bureau of Weights and Measures in Sèvres, France.
This object, known as the standard kilogram, had a height and diameter of about 39 millimeters (0.039 m), and was stored under carefully controlled conditions to ensure its mass remained stable.
However, even such a stable object could experience minuscule changes over time, prompting scientists to seek a more universal and lasting definition.
Since 2019, the kilogram has been redefined based on fundamental physical constants, most notably Planck’s constant (h).
This update marked a significant advance, as it relies on an unchanging quantity that can be replicated anywhere, without needing to reference a physical object.
The new definition is based on a complex equation that incorporates the speed of light in a vacuum (c) and the cesium frequency (fCs), expressed as: $$ 1\, kg = \frac{(299792458)^2 \cdot hf_{Cs}}{(6.62607015 \cdot 10^{-34})(9192631770)} $$ While this equation may seem complicated, its main purpose is to define the kilogram using fundamental constants that are immutable. In simpler terms, we can now “measure” the kilogram through quantum properties rather than by comparing it to a physical object. This brings greater accuracy and long-term stability, ensuring that measurements remain consistent anywhere in the universe, not just on Earth.
Mass vs. Weight: Two Distinct Concepts
It’s important not to confuse mass with weight. Although the two terms are often used interchangeably in everyday language, they refer to different concepts in physics.
- Mass is an intrinsic property of an object and does not change based on its location. An object with a mass of 10 kg on Earth will have the same mass on the Moon or anywhere else in the universe.
- Weight, however, is the gravitational force exerted on an object. It depends on the local gravity, which varies based on location. For example, on the Moon, where gravity is about one-sixth of that on Earth, an object will weigh less, but its mass will stay the same.
In short, weight is a force (measured in Newtons) that depends on both mass and the acceleration due to gravity.
The relationship between mass and weight is described by Newton’s second law:
$$ W = m \cdot g $$
Where $ W $ is weight, $ m $ is mass, and $ g $ is the gravitational acceleration (on Earth, approximately 9.81 m/s2).
Understanding the distinction between mass and weight is crucial in physics and applied sciences, as the two terms are often confused in everyday language but have very different meanings and practical implications.
