Unit of Mass - Definition, SI Unit for Mass, other Units, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:39 PM IST

What is a Mass Unit?

Mass can be considered as a physical asset and a measure of its resistance to acceleration when using a certain amount of energy. However, an important point the reader should keep in mind is that the mass is different from the weight even though the term can sometimes be used differently, as it has different units.

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What is a Mass Unit?
A. SI Mass Unit
B. CGS Mass Unit
Mass measurement

Unit of Mass - Definition, SI Unit for Mass, other Units, FAQs

The amount of matter in a given object is known as mass, and the units of measurement most commonly used are:

Gram
Kilograms

Mass can also be called the number of atoms and molecules in existence. Scientifically, the mass is measured in units of a metric system using kilograms (thousand grams), grams, centigrams (hundred grams), and milligrams (thousandth grams).

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A. SI Mass Unit

The given standard unit of weight is known as Kilogram (kg)

For SI units, the following units are accepted: gram (g) and its multiplication with submultiples, tonne (t) or metric ton, the simplest atomic mass unit (u) to show the abundance of atoms and molecules.

B. CGS Mass Unit

The second Gram centimeter, otherwise the CGS unit system is a metric system based on centimetres as a unit of length, the gram as a unit of mass, and second as a unit of time.

Other General Mass Units:

General Mass Units
Milligram (mg)	0.001 gram or 1/1000 gram
Decigram (dg)	0.1 gram or 1/10 gram
Centigram (cg)	0.01 gram or 1/100 gram
Gram (g)	One thousand milligrams
Decagram (dag)	10 grams
Hectogram (hg)	100 grams
Kilograms (kg)	1,000 grams

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Mass measurement

The amount of matter within an object is represented by the mass, but the mass of the object does not mean that you have measured the weight from the change in weight due to the effect of gravity. However, the mass does not change no matter where the object is, the value of the story remains the same. Depending on the mass and location of the objects, scientists use a variety of tools to measure mass.

The mass is measured by a balance device, also known as the mass scale, the balance scale, the simple scales, the weight balance, the weight scale, or the weight balance.

As mentioned above, there are many measuring instruments in various areas such as scales, measuring transducers, Newtonian measuring devices, vibrating tube sensors, and the use of gravity interactions between objects.

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A. Scales

By using everyday objects, scientists use the balance to determine the mass of an object. One example of measuring the balance of a pole three times. There are various types of balance, including beam measurements and digital scientific measurements. In space, scientists use an inertial balance to measure magnitude.

B. Transducer Rate

Sometimes mass cannot be determined by using balance. Transducers are used by scientists to measure the weight of a liquid in a measuring tank. It also measures the mass when the liquid structures are in a stable position. The signal is sent to the processor via a transducer, which allows quantity statistics to be indicated by the index. Taking the weight of the liquid under the transducer, and removing the pile of steam, the weight of the floating roof, the pile of the lower pillar and the water produces a great deal of weight.

C. Long Space Mass Measurement

It is one of the most widely used measurement instruments used to measure human weight in orbit, currently in the Earth's space station. It is a piece of cassette equipment based on Sir Isaac Newton's motion law, in which the force is equal to the product of gravity and acceleration. By using two human-powered springs, this device determines a person's weight in terms of strength and speed.

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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What are the units of mass and weight?

The kilogram is the SI unit of mass and it is the almost universally used standard mass unit. The associated SI unit of force and weight is the Newton.

2. Give some examples of Mass conversions.

Converting pounds into grams

1 kilogram = 1000 grams

To convert pounds into grams, we multiply by 1000 times the number of pounds.

Example:

7 pounds = 7 × 1000 = 7000 grams

Converting grams into pounds

1 gram = 1000 kilograms

By dividing one kilogram from 1000, we get a conversion in grams

Example:

8g = 8 ÷ 1000 = 0.008 kg

3. If Kg is a Unit of Mass, then What is a SI Unit of Weight?

The given SI unit of weight is Newton

According to the following equation

F = ma

We know that the weight of an object is too heavy for it

Therefore, F = Weight

As mentioned above,

SI unit weight = Force = Newton (N)

4. What is SI unit of Mass?

Mass of substances in the body regardless of their volume or any forces acting on it. The SI Mass unit is Kilogram (Kg) and the CGS is gram (g).

5. What is the Mass formula?

Definition of Mass: Mass is defined as the number of things that exist in the body. Mass = Density × Volume.

6. What is the SI unit of mass and why was it chosen?

The SI unit of mass is the kilogram (kg). It was chosen as the base unit for mass because it represents a convenient and practical amount of mass for everyday use, while also being precisely definable for scientific purposes. Originally, it was defined as the mass of 1 liter of water at 4°C, but now it's defined based on the Planck constant.

7. Why is the kilogram the only SI base unit with a prefix?

The kilogram is unique among SI base units in having a prefix (kilo-) as part of its name. This is a historical artifact from when the gram was initially considered as the base unit. The kilogram was later chosen as the standard because it was a more practical size for many measurements, but the name was retained to avoid confusion.

8. How does the definition of the kilogram relate to the Planck constant?

The modern definition of the kilogram is based on the Planck constant (h), a fundamental constant of nature. The kilogram is defined by taking the fixed numerical value of h to be 6.62607015 × 10⁻³⁴ when expressed in the unit J⋅s, which is equal to kg⋅m²⋅s⁻¹. This definition allows the kilogram to be realized using a Kibble balance, which relates mass to electromagnetic force and the Planck constant.

9. What is the smallest unit of mass that physicists work with?

The smallest unit of mass that physicists commonly work with is the electron mass, approximately 9.1 × 10⁻³¹ kg. In particle physics, even smaller masses are encountered, such as those of neutrinos, but these are typically expressed in energy units (eV/c²) rather than mass units due to the mass-energy equivalence principle.

10. What is the difference between atomic mass and molecular mass?

Atomic mass is the mass of a single atom of an element, usually expressed in atomic mass units (u). Molecular mass is the sum of the atomic masses of all atoms in a molecule. For example, the atomic mass of hydrogen is about 1 u, while the molecular mass of H₂O (water) is about 18 u (2 × 1 u for hydrogen + 16 u for oxygen).

11. How do scientists measure extremely small masses?

For extremely small masses, scientists use various techniques:

12. How do scientists determine the mass of planets and stars?

Scientists use several methods to determine the mass of celestial bodies:

13. How does mass relate to momentum?

Momentum (p) is the product of an object's mass (m) and its velocity (v): p = mv. This means that for a given velocity, an object with more mass will have more momentum. Conversely, to achieve the same momentum, a less massive object would need to move at a higher velocity than a more massive object.

14. What is the difference between average atomic mass and isotopic mass?

Isotopic mass is the mass of a specific isotope of an element, which has a defined number of neutrons. Average atomic mass is the weighted average of the masses of all naturally occurring isotopes of an element, taking into account their relative abundances. For example, chlorine has two main isotopes (³⁵Cl and ³⁷Cl), and its average atomic mass (about 35.45 u) reflects the weighted average of these isotopes' masses.

15. What is the relationship between mass and gravitational potential energy?

Gravitational potential energy (U) is directly proportional to mass (m). Near Earth's surface, it's given by U = mgh, where g is the acceleration due to gravity and h is the height above a reference point. This means that for a given height, an object with more mass will have more gravitational potential energy.

16. How does mass differ from weight?

Mass is a measure of the amount of matter in an object, while weight is the force exerted on that mass due to gravity. Mass remains constant regardless of location, but weight can change depending on the strength of the gravitational field. For example, an object's mass would be the same on Earth and the Moon, but its weight would be different.

17. Why doesn't mass change with location like weight does?

Mass is an intrinsic property of matter, representing the amount of matter in an object. It's independent of external factors like gravity or location. Weight, on the other hand, is a force that depends on both mass and the local gravitational field strength, which can vary with location.

18. What's the difference between mass and density?

Mass is the total amount of matter in an object, measured in units like kilograms. Density is the mass per unit volume, typically measured in kg/m³. While mass tells you how much matter is present, density tells you how compact or spread out that matter is within a given space.

19. What are some common non-SI units of mass, and how do they relate to the kilogram?

Common non-SI units include:

20. Can mass ever be negative?

In classical physics, mass cannot be negative. Mass represents the amount of matter in an object, which is always positive or zero. However, in some areas of theoretical physics, like certain quantum field theories, the concept of negative mass is explored, but this remains purely theoretical and hasn't been observed in nature.

21. How is mass related to inertia?

Mass and inertia are directly related. Inertia is an object's resistance to changes in its state of motion. The more mass an object has, the greater its inertia. This means that objects with more mass require more force to accelerate or decelerate than objects with less mass.

22. What is the concept of relativistic mass and how does it differ from rest mass?

Rest mass (m₀) is the mass of an object when it's not moving relative to the observer. Relativistic mass (m) is the apparent increase in an object's mass as it approaches the speed of light, given by m = γm₀, where γ is the Lorentz factor. However, modern physicists prefer to use only the concept of rest mass and express the relativistic effects in terms of energy and momentum rather than a changing mass.

23. What is meant by "massless particles" in physics?

"Massless particles" are particles that have no rest mass but still carry energy and momentum. The most well-known massless particle is the photon, the particle of light. Gluons, which mediate the strong nuclear force, are also considered massless. These particles always travel at the speed of light in vacuum and their energy is entirely in the form of momentum.

24. How does gravity affect objects with different masses?

Gravity affects all objects with mass, but the force of gravity (weight) is proportional to the mass. This means that objects with greater mass experience a stronger gravitational force. However, in the absence of air resistance, all objects fall at the same rate regardless of their mass. This is because the greater gravitational force on more massive objects is exactly balanced by their greater inertia.

25. What is the relationship between mass and volume?

Mass and volume are related through density. Density is defined as mass per unit volume (ρ = m/V). For a given material, if you know its density and either its mass or volume, you can calculate the other property. However, mass and volume are not directly proportional for different materials, as their densities can vary greatly.

26. How does mass relate to energy according to Einstein's famous equation?

Einstein's equation E = mc² relates mass (m) to energy (E), where c is the speed of light in vacuum. This equation shows that mass and energy are equivalent and can be converted into each other. It implies that even a small amount of mass contains an enormous amount of energy, which is the basis for nuclear energy and explains the sun's energy output.

27. How does the conservation of mass principle relate to chemical reactions?

The conservation of mass principle states that in a closed system, the total mass remains constant before and after a chemical reaction. This means that while atoms may be rearranged in a reaction, no mass is created or destroyed. In practice, this principle is often observed as the conservation of matter, since some mass can be converted to energy (or vice versa) in nuclear reactions, but this effect is negligible in chemical reactions.

28. How does mass affect the center of gravity of an object?

The center of gravity is the average location of the weight of an object. It's affected by both the distribution of mass within the object and the total mass. In a uniform gravitational field, the center of gravity coincides with the center of mass. Objects with unevenly distributed mass will have their center of gravity shifted towards the more massive parts, which can affect stability and balance.

29. What is meant by "reduced mass" in physics?

Reduced mass is a concept used in two-body problems, such as in orbital mechanics or molecular vibrations. It's defined as μ = (m₁m₂)/(m₁ + m₂), where m₁ and m₂ are the masses of the two bodies. The reduced mass simplifies calculations by allowing a two-body problem to be treated as a one-body problem, making it easier to solve equations of motion.

30. What is the concept of critical mass in nuclear physics?

Critical mass is the minimum amount of fissile material needed to maintain a nuclear chain reaction. When the mass of fissile material exceeds the critical mass, it can sustain a chain reaction without external neutron sources. This concept is crucial in both nuclear power generation and nuclear weapons. The critical mass depends on factors like the type of material, its purity, and its geometry.

31. How does mass affect the period of a simple pendulum?

Interestingly, the mass of the bob (the weight at the end of a pendulum) does not affect the period of a simple pendulum. The period depends only on the length of the pendulum and the local gravitational field strength. This is because the greater gravitational force on a more massive bob is exactly balanced by its greater inertia, resulting in the same period regardless of mass.

32. How does mass affect the escape velocity from a celestial body?

Escape velocity is the minimum speed an object needs to escape a celestial body's gravitational field without further propulsion. It's given by v = √(2GM/R), where G is the gravitational constant, M is the mass of the celestial body, and R is its radius. This shows that bodies with larger mass have higher escape velocities, making it harder for objects to leave their gravitational influence.

33. How does the concept of mass apply to subatomic particles?

At the subatomic level, mass is a more complex concept. Particles like protons and neutrons derive most of their mass from the energy of the quarks and gluons within them, rather than from the masses of their constituent quarks. This is described by quantum chromodynamics. Some particles, like electrons, have intrinsic mass, while others, like photons, are massless. The origin of particle masses is explained by the Higgs mechanism in the Standard Model of particle physics.

34. What is meant by "mass defect" in nuclear physics?

Mass defect is the difference between the mass of an atomic nucleus and the sum of the masses of its constituent nucleons (protons and neutrons). This mass difference is converted to binding energy according to Einstein's E = mc² equation. The binding energy represents the energy required to break the nucleus apart into its constituent nucleons. A larger mass defect indicates a more stable nucleus.

35. What is the concept of effective mass in solid-state physics?

Effective mass is a quantity used to describe the behavior of electrons (or holes) in a crystal lattice. It takes into account how the periodic potential of the crystal affects the electron's response to forces, making it appear as if the electron has a different mass than its actual mass. The effective mass can be positive, negative, or even infinite, depending on the band structure of the material. This concept is crucial for understanding electrical properties of semiconductors and other materials.

36. What is the role of mass in the classification of stars?

Mass plays a crucial role in determining a star's life cycle and classification. More massive stars burn their fuel faster, are hotter and brighter, and have shorter lifespans than less massive stars. The mass of a star determines its position on the Hertzsprung-Russell diagram, its fusion processes, and its ultimate fate (e.g., white dwarf, neutron star, or black hole). Stellar classification systems like the Harvard spectral classification are indirectly based on mass through its effects on temperature and luminosity.

37. How does mass relate to buoyancy?

Buoyancy is the upward force exerted by a fluid on an immersed object. While buoyancy itself doesn't depend directly on the object's mass, it's related to the mass of fluid displaced by the object. An object will float if its mass is less than the mass of the fluid it displaces. This relationship is described by Archimedes' principle: the buoyant force is equal to the weight of the displaced fluid.

38. What is meant by "invariant mass" in special relativity?

Invariant mass, also called rest mass, is the mass that an object has when it is at rest relative to the observer. In special relativity, this is the only mass concept that remains constant for all observers, regardless of their relative motion. The invariant mass is related to the total energy and momentum of a system by E² = (mc²)² + (pc)², where E is total energy, m is invariant mass, c is the speed of light, and p is momentum.

39. What is the difference between inertial mass and gravitational mass?

Inertial mass is the mass that appears in Newton's second law of motion (F = ma), representing an object's resistance to acceleration. Gravitational mass is the mass that appears in Newton's law of gravitation, representing an object's response to gravitational force. While these are conceptually different, they are experimentally observed to be equivalent (the equivalence principle), which is a fundamental assumption in Einstein's theory of general relativity.

40. How does mass affect the de Broglie wavelength of a particle?

The de Broglie wavelength (λ) of a particle is given by λ = h/p, where h is Planck's constant and p is the particle's momentum. Since momentum is the product of mass and velocity (p = mv), for a given velocity, particles with larger mass will have shorter de Broglie wavelengths. This is why quantum effects are more noticeable for lighter particles like electrons than for more massive objects.

41. How does mass affect the moment of inertia of an object?

Moment of inertia (I) is a measure of an object's resistance to rotational acceleration. It depends on both the mass of the object and how that mass is distributed relative to the axis of rotation. For a point mass, I = mr², where m is the mass and r is the distance from the axis of rotation. For extended objects, the moment of inertia is calculated by integrating over the entire mass distribution. Generally, objects with more mass or mass distributed further from the axis of rotation have larger moments of inertia.

42. How does mass relate to the concept of center of mass?

The center of mass is the average position of mass in a system. For a system of particles, it's calculated as r_cm = Σ(m_i * r_i) / Σm_i, where m_i is the mass of each particle and r_i is its position. The center of mass behaves as if all the mass of the system were concentrated at that point for many calculations involving external forces. In a uniform gravitational field, the center of mass coincides with the center of gravity.

43. How does the concept of mass apply to black holes?

For black holes, mass determines the size of the event horizon (the boundary beyond which nothing can escape) according to the Schwarzschild radius: R = 2GM/c², where G is the gravitational constant, M is the black hole's mass, and c is the speed of light. The mass of a black hole can be inferred from its