Definition · Plain-language
Mass
Mass is the amount of matter in an object and a measure of its resistance to being accelerated — a property that stays the same wherever the object is.
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The amount of matter
Mass measures how much matter an object contains. A bowling ball has more mass than a tennis ball because it is made of more, and denser, material. This quantity is intrinsic to the object: it does not depend on where the object is, what it is resting on, or whether any gravity acts on it at all. Mass is measured in kilograms, the SI base unit for the quantity, and it is a scalar — it has size but no direction. This is the everyday sense of mass: a fixed measure of "how much stuff" is there.
Mass as resistance to acceleration
There is a second, deeper way to understand mass: it measures inertia, an object’s resistance to a change in its motion. The more mass something has, the harder it is to start it moving, stop it or change its direction — captured by Newton’s law that force equals mass times acceleration. A loaded trolley is harder to push than an empty one because it has more mass and therefore more inertia. Remarkably, this inertial mass and the mass that responds to gravity turn out to be the same quantity, a fact at the heart of modern physics.
Mass versus weight
Mass is often confused with weight, but they are different. Mass is the amount of matter, measured in kilograms and constant everywhere. Weight is the gravitational force on that mass, measured in newtons, and it changes with the local strength of gravity — an object weighs about six times less on the Moon than on Earth, though its mass is unchanged. Everyday scales blur the distinction by displaying mass in kilograms while actually responding to weight. In careful scientific use the two are always kept apart.
Key facts
At a glance
- Definition: the amount of matter in an object and its resistance to acceleration
- SI unit: the kilogram (kg)
- Constant: the same on Earth, the Moon or in space
- Quantity type: a scalar — size only, no direction
- Relation: weight = mass × gravitational acceleration
- Since 2019: the kilogram is defined by fixing the Planck constant
Common misconceptions
What people often get wrong
Often heard: Mass and weight are the same thing.
Actually: They are not. Mass is the amount of matter, measured in kilograms and unchanging; weight is the gravitational force on that mass, measured in newtons and varying with gravity.
Often heard: An object loses mass in space because it becomes weightless.
Actually: Weightlessness means there is no gravitational force, so weight is zero — but the matter is still present, so the mass is completely unchanged.
Often heard: Mass is only about how much something weighs on a scale.
Actually: Mass also measures inertia — resistance to acceleration. A massive object is hard to push or stop regardless of gravity, which is why force equals mass times acceleration.
Going deeper
