This entry was posted on January 10, 2024 by Anne Helmenstine (updated on January 13, 2024)

Mass is a fundamental concept in science. Basically, it is the amount of matter in an object. It’s an intrinsic property that is independent of an object’s location or the arrangement of the matter within it. Unlike weight, mass doesn’t change when an object’s position changes.

### Mass Definition

- The simple definition of mass is that it is the amount of matter in an object.
- A more precise definition is that it is a measure of inertia or a body’s resistance to change in response to a force.

In scientific terms, mass is a measure of an object’s resistance to acceleration when a force is applied. It also determines the strength of its mutual gravitational attraction with other bodies. Under ordinary circ*mstances, the mass of an object is constant and does not change unless the object itself changes by gaining or losing matter.

### Units of Mass

The basic unit of mass in the International System of Units (SI) is the kilogram (kg). Other units include grams (g), milligrams (mg), and metric tons. In the United States, mass is often measured in pounds (lb), although this is a unit of weight.

### Mass vs. Weight

The distinction between mass and weight is crucial. Mass is a measure of how much matter an object contains, while weight is the force exerted by gravity on that mass. Essentially, weight is the product of mass and the acceleration due to gravity. This means that an object’s weight changes depending on where it is in the universe, but its mass remains the same. On Earth, gravity is nearly constant, so mass and weight are often used interchangeably.

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### Scientific Laws and Formulas Involving Mass

Mass is a key component in several fundamental scientific laws and equations:

**Newton’s Second Law of Motion**: This law states that force equals mass times acceleration (F = ma).**Law of Universal Gravitation**: The gravitational force between two masses is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.**Einstein’s Mass-Energy Equivalence**: Expressed as E=mc², this equation suggests that mass can be converted into energy and vice versa. This principle is fundamental in nuclear physics and cosmology.

### Importance in Science and Everyday Life

In science, mass is a cornerstone in fields such as physics, chemistry, and engineering. It helps in understanding the behavior of objects under different forces and conditions. For instance, in chemistry, the mass of substances determines the stoichiometry for reaction equations.

In everyday life, mass is everywhere:

**Cooking and Baking**: Recipes require precise measurements of ingredients, typically in grams or ounces.**Healthcare**: Dosages of medication rely on mass, considering the patient’s body mass.**Engineering and Construction**: The mass of materials is part of the calculations for stability and integrity in structures.**Transportation**: The mass of vehicles and their loads are important in safety and efficiency.

### Measuring Mass

Determining mass is a fundamental process in both scientific experiments and everyday applications. Here are some common ways of measuring mass:

**Balancing Scales:**These scales compare the unknown mass to a known standard. The most classic example is the beam balance. When the scale is in equilibrium (balanced), the mass on one side is equal to the mass on the other. This method directly measures mass and is independent of gravity.**Weighing Scales:**These scales measure the force exerted by an object (due to gravity) and then calculate the mass based on the gravitational force. The reading varies slightly depending on the local gravitational field, which changes with location on the Earth’s surface.**Inertial Mass Measurement:**This method determines mass by measuring an object’s resistance to acceleration when a known force is applied and using Newton’s second law of motion (*F = ma*, where*F*is force,*m*is mass, and*a*is acceleration). Rearranging the formula,*m = F/a*.**Gravitational Mass Measurement:**This method involves measuring the force of attraction between the object and the Earth (or another known mass). The calculation solves for mass using Newton’s law of universal gravitation.**Volume and Density Method:**Mass equals density multiplied by volume. So, scientists measure the volume of an object with a known density and solve for mass.**Oscillation Method:**The value for mass comes from observing the frequency of oscillation in a known medium.**Mass Spectrometry:**This method measures the mass-to-charge ratio of ionized chemical compounds of known composition.**Atomic Force Microscopy:**This method measures the mass of nano-scale particles.**Astronomical Methods:**Mass estimates comes from observation of gravitational effects on orbital mechanics of planets or stars.

### Imaginary Mass

The idea of imaginary mass is part of the effort to extend or go beyond the Standard Model of particle physics, particularly in exploring the nature of dark matter and dark energy. It’s important to note that “imaginary mass” does not refer to a physical mass in the conventional sense, but rather to a mathematical formulation that has implications for the behavior of particles and the structure of spacetime.

In physics, when equations yield solutions involving the square root of a negative number (and thus imaginary numbers), it often indicates something profound or unusual about the system being studied. Here’s how the concept applies to mass:

**Tachyons**: Imaginary mass relates to hypothetical particles called tachyons. Theoretically, tachyons travel faster than the speed of light. In the framework of Einstein’s Special Relativity, a particle with real, positive mass only reaches the speed of light if it has infinite energy, which is impossible. However, if a particle has imaginary mass, the equations suggest it can naturally exist at speeds greater than light.**Quantum Field Theory**: Imaginary mass also appears in quantum field theory, particularly in discussions about particle fields. It’s often a sign of instability in a field or a need for the field to undergo a process called “symmetry breaking,” which is crucial in the Standard Model of particle physics.

### References

- Hecht, Eugene (2006). “There Is No Really Good Definition of Mass”.
*The Physics Teacher*. 44 (1): 40–45. doi:10.1119/1.2150758 - Okun, L.B. (2002). “Photons, Clocks, Gravity and the Concept of Mass”.
*Nuclear Physics B: Proceedings Supplements*. 110: 151–155. doi:10.1016/S0920-5632(02)01472-X - von Klitzing, Klaus (2017). “Metrology in 2019“.
*Nature Physics*. 13 (2): 198.