Mass is a property of matter. It doesn’t change, even if an object changes its shape or position in the universe. This is what makes it different from weight, which is the force of gravity on an object.
The kilogram is one of the seven SI base measurement units. It was once anchored to a physical prototype, but the definition of the kilogram now is based on fundamental physical constants.
Inertial mass
Inertial mass is a physical quantity that measures inertia. The fact that inertial mass and gravitational mass are equal was a major intuition that led Einstein to formulate the theory of General Relativity. He postulated that gravity was not a force but a warping of spacetime. Since then, many experiments have been performed to verify this hypothesis, and they have all agreed with each other.
Inertial mass is also known as an object’s resistance to acceleration, which can be calculated by dividing the ratio of the force applied to the mass. The physics behind this concept is simple: an object that has more mass will be harder to push than one with less mass. This is why we use the metric system, which defines the units pound-mass for mass and pound-force for force. However, in countries using the imperial system, this causes problems when referring to weight and mass.
Active gravitational mass
A measurement of an object’s active gravitational mass determines the strength of its reaction to a gravity field. This is different from inertial mass, which determines how easily an object accelerates in response to a force. In general, objects with more active gravitational mass are closer to the center of a gravity field and therefore have a stronger reaction.
Until Newton, physicists didn’t make much of a distinction between the three types of mass. They were all thought to be equivalent. Even today, when a physicist uses a balance to weigh a sample of aluminum and iron, they assume the forces exerted on each sample by the gravitational fields are equal.
Invariant mass, or rest-mass, is the true count of an object’s potential and kinetic energy in a special resting frame. It’s also what we use to calculate the inverse of speed, E=MC2. In other words, it gives us a true picture of the shenanigans that energy is up to.
Passive gravitational mass
In the context of Newtonian gravity, it is important to distinguish between inertial mass and passive gravitational mass. These two concepts are related to each other but differ in how they are measured. Inertial mass, or rest-mass, is defined by comparing an object with a fixed reference frame of motion, while passive gravitational mass is defined by measuring the force of an object within a free-falling frame. Using the metric system for measurement, an object with greater inertial mass will accelerate more quickly than an object with lesser inertial mass.
Passive gravitational mass is also used to determine the strength of a body’s gravitational field. This is the force that causes objects to attract each other. The equivalence of inertial and passive gravitational masses is an important aspect of the symmetry of gravity, which is a fundamental principle in general relativity. New research has confirmed this equivalence to higher precision than ever before. This is good news for scientists.
Equivalent mass
The equivalent mass of an element is the amount of an element that combines perfectly with a certain quantity of another substance. It is used in chemical reactions to determine the molar mass of a compound or ion. The molar mass of an ion is equal to its total mass divided by the number of protons and neutrons.
Unlike atomic mass, which is dimensionless, equivalent mass has a unit of measurement: the gram-equivalent (g eq) value. The gram-equivalent value of an element is the amount of an element’s atoms that combine with or displace 1.008 grams of hydrogen, 8 grams of oxygen, or 35.5 grams of chlorine.
In the rest frame of a moving system, mass and energy are linked by Einstein’s formula E = mc2, where mass m is the rest mass and energy E is the product of the mass times the speed of light squared. This principle is called the law of mass-energy equivalence.