What is Mass?
Mass is the fundamental measure of how much matter an object contains. Unlike weight, mass never changes regardless of where you are in the universe.
Definition of Mass
In physics, mass is a scalar quantity that represents the amount of matter in an object. It is one of the seven fundamental physical quantities in the International System of Units (SI), with the kilogram (kg) as its base unit.
Mass has two key manifestations: inertial mass (resistance to acceleration when a force is applied) and gravitational mass (the property that determines gravitational attraction). Einstein's equivalence principle states these are identical.
Key Fact
Since 2019, the kilogram is defined by fixing the Planck constant at exactly h = 6.62607015 × 10⁻³⁴ J·s, no longer relying on a physical platinum-iridium cylinder.
Mass vs Weight: Key Differences
Mass and weight are often confused in everyday language, but they are fundamentally different physical concepts:
| Property | Mass | Weight |
|---|---|---|
| Definition | Amount of matter | Gravitational force on mass |
| SI Unit | Kilogram (kg) | Newton (N) |
| Type | Scalar quantity | Vector quantity |
| Changes with location? | No — constant everywhere | Yes — depends on gravity |
| Measured with | Balance | Spring scale |
| Formula | m = F / a | W = m × g |
Example
A 70 kg person weighs 686 N on Earth (70 × 9.81), but only 114 N on the Moon (70 × 1.62). Their mass remains 70 kg in both places.
Common Units of Mass
| Unit | Symbol | Equivalent in kg | Usage |
|---|---|---|---|
| Milligram | mg | 0.000001 | Pharmaceuticals, chemistry |
| Gram | g | 0.001 | Cooking, nutrition labels |
| Kilogram | kg | 1 | SI base unit, everyday use |
| Metric ton | t | 1,000 | Industry, shipping |
| Ounce (avoirdupois) | oz | 0.02835 | US/UK retail, food |
| Pound | lb | 0.45359 | US/UK body weight, retail |
| Stone | st | 6.35029 | UK body weight |
| Atomic mass unit | u (amu) | 1.661 × 10⁻²⁷ | Atomic/molecular physics |
History of Mass Measurement
Ancient Balances
Egyptians used stone and metal counterweights with beam balances for trade. The oldest known unit was the 'deben' (~91 grams).
Metric System Born
France introduced the metric system. The 'gramme' was defined as the mass of 1 cubic centimeter of water at 4°C.
International Prototype Kilogram
A platinum-iridium cylinder (IPK) stored in Paris became the world's standard kilogram for 130 years.
SI System Established
The International System of Units (SI) was adopted, with the kilogram as the base unit of mass.
Planck Constant Redefinition
The kilogram was redefined in terms of the Planck constant, eliminating dependence on a physical artifact for the first time.
Your Weight on Other Planets
If your mass is 70 kg, here's what you would weigh (gravitational force) on other celestial bodies:
Moon
113 N
g = 1.62 m/s²
Mars
260 N
g = 3.72 m/s²
Jupiter
1,735 N
g = 24.79 m/s²
Saturn
731 N
g = 10.44 m/s²
Venus
621 N
g = 8.87 m/s²
Mercury
259 N
g = 3.7 m/s²
Earth
686 N
g = 9.81 m/s²
Sun
19,180 N
g = 274 m/s²
Frequently Asked Questions
Common questions about mass, weight, and measurement
What is the definition of mass?
Mass is a fundamental property of matter that quantifies the amount of material in an object. It is measured in kilograms (kg) in the SI system and remains constant regardless of location — unlike weight, which depends on gravitational force.
What is the difference between mass and weight?
Mass measures the quantity of matter (in kg) and stays constant everywhere. Weight measures gravitational force on that mass (in Newtons) and changes with location. On the Moon, your mass is the same but your weight is about 1/6th of Earth's weight.
What is the SI unit of mass?
The kilogram (kg) is the SI base unit of mass. It was redefined in 2019 using the Planck constant (h = 6.62607015 × 10⁻³⁴ J·s), making it independent of any physical artifact.
Why does mass stay the same on different planets?
Mass is an intrinsic property of matter — it depends on the number and type of atoms in an object, not on gravity. Weight (force) changes because gravitational acceleration varies between planets, but the actual matter remains unchanged.
How is mass measured?
Mass is measured using a balance (comparing against known masses) or a digital scale calibrated in kilograms. In physics, mass can also be determined via its inertia — resistance to acceleration under a known force (F = ma).
What is the relationship between mass and energy?
Einstein's famous equation E = mc² establishes that mass and energy are equivalent. The energy content of matter equals its mass multiplied by the speed of light squared. This principle underlies nuclear energy and particle physics.