Short answer
Mass describes an object’s inertia; weight, in the physics sense used here, is the gravitational force acting on it. Move the same object from Earth to the Moon and its mass stays the same, while its weight changes because gravity is weaker there. The SI unit of mass is the kilogram; the SI unit of weight is the newton. 1 2
Everyday “weight” often means mass, including when someone records a weight in kilograms. There is also apparent weight: the support force associated with what a scale reads. That distinction explains how orbiting astronauts can feel weightless while gravity still acts on them. 1 2 3
On this page
At a glance
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| Question | Mass | Gravitational weight |
|---|---|---|
| What does it describe? | Inertia: resistance to a change in motion. | A force due to gravity. |
| SI unit | Kilogram, kg. | Newton, N. |
| Does moving to the Moon change it? | Not merely because the location changed. | Yes, because the gravitational field differs. |
| Is direction part of the quantity? | No; mass is a scalar. | Yes; gravitational force has direction. |
| Is it zero during orbital free fall? | No. | No; gravity still provides the orbital acceleration. |
| What is the connection? | Supplies the mass in the calculation. | In the simple local model, weight magnitude is W = mg. |
Sources: definitions and units 1 2; orbital free fall 3. Scope: ordinary, introductory Newtonian mechanics.
What each thing is
Mass
Mass measures an object’s inertia. In Newton’s second law, a given net force produces less acceleration in an object with greater mass. Saying mass is “how much matter” is a useful starting description, but inertia explains why it appears in calculations about motion. 1 2
Weight
Gravitational weight is the force exerted on a mass by a gravitational field. Near Earth’s surface its magnitude is commonly calculated as W = mg, where m is mass and g is local gravitational acceleration, approximately 9.8 meters per second squared. 2
An unchanged mass can therefore have different weights in different gravitational fields. The equation relates the quantities; it does not make them interchangeable.
Key differences
Mass belongs to the object in this model; weight also depends on location. Earth’s gravitational acceleration varies somewhat across its surface, and differs much more from the Moon’s. A specified mass alone does not establish gravitational weight everywhere. 2
The units describe different quantities. Kilograms measure mass. Newtons measure force. In US usage, pounds also appear in force descriptions; readers should establish whether a statement is using everyday “weight” or a physics quantity before interpreting it. 1 2
Feeling heavy involves support. A floor or scale pushes on an object to support it. During orbital free fall, a spacecraft and its occupants fall together, so occupants can float without the usual supporting force. Gravity has not disappeared. 2 3
How to tell them apart
Look first at the question and units. “How many kilograms?” asks for mass. “What force does gravity exert?” asks for weight. To convert a mass to gravitational weight, supply the relevant value of g. 1 2
If the question concerns a scale reading, an elevator, or an astronaut feeling weightless, establish whether it means apparent weight. A scale reading and gravitational force need not coincide during acceleration or free fall. Treating the instrument’s display as a complete physical definition can produce the wrong explanation. 2 3
Where they overlap
At a fixed location, gravitational weight is proportional to mass. That close everyday relationship makes it easy to use the words interchangeably. A heavier mass sitting beside a lighter one is also acted on by a larger gravitational force, but changing the gravitational field exposes the distinction. 2
Edge cases
Orbiting is not escaping gravity. Gravity curves an orbiting object’s path. The familiar weightless experience concerns free fall and the absence of ordinary support, rather than zero mass or zero gravitational attraction. 3
A scale can display kilograms. Everyday weighing can report an estimate of mass even though the apparatus responds to a force. The display unit and the quantity being inferred matter. NIST explicitly recognizes this everyday use of “weigh.” 1
“Mass never changes” is too broad. Moving an otherwise unchanged object does not alter its mass in the comparison here. Adding or removing material plainly can. This article is not a treatment of relativistic mass-energy bookkeeping. 2
Why the distinction exists
Mechanics needs to distinguish an object’s response to forces from one particular force acting on it. Mass belongs in the general force-and-acceleration relationship; weight describes the gravitational interaction. On Earth the two track each other closely enough that ordinary speech often blends them. 1 2
Common misconceptions
- “Kilograms are units of force.” They are units of mass. 1
- “A person loses mass on the Moon.” Relocation alone changes gravitational weight. 2
- “Weightlessness means no gravity.” Orbital free fall supplies the counterexample. 3
- “A scale always reads gravitational force directly.” Support conditions and the displayed units matter. 1 3
Examples
Using rounded introductory values, a 10 kg object has a gravitational weight of about 98 N on Earth when g is 9.8 m/s². On the Moon, using 1.62 m/s², it weighs about 16.2 N. Its mass remains 10 kg. These are illustrative calculations using W = mg, not measurements of a particular object or location. 2
Sources
Sources checked October 3, 2026.
- NIST — SI Units: Mass. Mass/weight FAQ and units; the article relies on these sections, not historical artifact descriptions elsewhere on the page.
- OpenStax, University Physics Volume 1 — Mass and Weight. Newtonian definitions, W = mg, and lunar comparison.
- NASA Glenn Research Center — What is Microgravity?. Orbital free fall and the weightless experience.