Mass versus weight

weightdistinctionhistorical conflation of mass and weightmass and weightmass or weightsingle-pan
In common usage, the mass of an object is often referred to as its weight, though these are in fact different concepts and quantities.wikipedia
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Mass

inertial massgravitational massweight
In common usage, the mass of an object is often referred to as its weight, though these are in fact different concepts and quantities.
But because of slight differences in the strength of the Earth's gravitational field at different places, the distinction becomes important for measurements with a precision better than a few percent, and for places far from the surface of the Earth, such as in space or on other planets.

Pound (mass)

lbpoundspound
For example, in retail commerce, the "net weight" of products actually refers to mass, and is expressed in mass units such as grams or ounces (see also Pound: Use in commerce). Similarly, the avoirdupois pound, used in both the Imperial system and U.S. customary units, is a unit of mass, and its related unit of force is the pound-force.
Usage of the unqualified term pound reflects the historical conflation of mass and weight.

Kilogram

kgmgmilligram
In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the surface of the Earth (its mass multiplied by the gravitational field strength), since the newton is a unit of force, while the kilogram is a unit of mass. The object's weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass. Under Sir Isaac Newton's 332-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.
The Kibble balance (known as a "watt balance" before 2016) is essentially a single-pan weighing scale that measures the electric power necessary to oppose the weight of a kilogram test mass as it is pulled by Earth's gravity.

Imperial units

imperialimperial systemimperial unit
Similarly, the avoirdupois pound, used in both the Imperial system and U.S. customary units, is a unit of mass, and its related unit of force is the pound-force.
The distinction between mass and weight is not always clearly drawn.

Weighing scale

scalesscalebalance
When one stands on a balance-beam-type scale at a doctor’s office, they are having their mass measured directly.
Mass versus weight

Pound (force)

lbflb f pounds-force
Similarly, the avoirdupois pound, used in both the Imperial system and U.S. customary units, is a unit of mass, and its related unit of force is the pound-force.
Mass versus weight for the difference between the two physical properties

Weight

gross weightweighingweigh
In common usage, the mass of an object is often referred to as its weight, though these are in fact different concepts and quantities.

Matter

corporealsubstancematerial
In scientific contexts, mass is the amount of "matter" in an object (though "matter" may be difficult to define), whereas weight is the force exerted on an object by gravity.

Force

forcesattractiveforce vector
In scientific contexts, mass is the amount of "matter" in an object (though "matter" may be difficult to define), whereas weight is the force exerted on an object by gravity.

Earth

terrestrialworldGlobal
In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the surface of the Earth (its mass multiplied by the gravitational field strength), since the newton is a unit of force, while the kilogram is a unit of mass. The object's weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass.

Gravity

gravitationgravitationalgravitational force
In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the surface of the Earth (its mass multiplied by the gravitational field strength), since the newton is a unit of force, while the kilogram is a unit of mass. The object's weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass. In scientific contexts, mass is the amount of "matter" in an object (though "matter" may be difficult to define), whereas weight is the force exerted on an object by gravity.

Newton (unit)

kNnewtonN
In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the surface of the Earth (its mass multiplied by the gravitational field strength), since the newton is a unit of force, while the kilogram is a unit of mass. The object's weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass. Under Sir Isaac Newton's 332-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.

Mars

Martianplanet MarsRed Planet
In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the surface of the Earth (its mass multiplied by the gravitational field strength), since the newton is a unit of force, while the kilogram is a unit of mass. The object's weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass.

Saturn

Saturn's atmosphereExploration of Saturnhome planet
In other words, an object with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the surface of the Earth (its mass multiplied by the gravitational field strength), since the newton is a unit of force, while the kilogram is a unit of mass. The object's weight will be less on Mars (where gravity is weaker), more on Saturn, and negligible in space when far from any significant source of gravity, but it will always have the same mass.

Buoyancy

buoyantbuoyant forcefloat
Similarly, a balloon has mass but may appear to have no weight or even negative weight, due to buoyancy in air.

Inertia

inertialinertia forceinertial forces
A better scientific definition of mass is its description as being composed of inertia, which is the resistance of an object being accelerated when acted on by an external force.

Theory of relativity

relativityrelativisticrelativity theory
While the weight of an object varies in proportion to the strength of the gravitational field, its mass is constant (ignoring relativistic effects) as long as no energy or matter is added to the object.

Isaac Newton

NewtonSir Isaac NewtonNewtonian
Under Sir Isaac Newton's 332-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.

Newton's laws of motion

laws of motionNewton's second lawNewton's second law of motion
Under Sir Isaac Newton's 332-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.

Acceleration

decelerationacceleratem/s 2
Under Sir Isaac Newton's 332-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.

Metre per second squared

m/s 2 meters per second squaredm/s²
Under Sir Isaac Newton's 332-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.

Gravity of Earth

ggravityEarth's gravity
Under Sir Isaac Newton's 332-year-old laws of motion and an important formula that sprang from his work, F = ma, an object with a mass, m, of one kilogram will accelerate, a, at one meter per second per second (about one-tenth the acceleration due to earth’s gravity) when acted upon by a force, F, of one newton.

Bowling ball

bowling ballsballbowling ball coverstock
Inertia is seen when a bowling ball is pushed horizontally on a level, smooth surface, and continues in horizontal motion.

Inelastic collision

inelasticinelastic collisionsinelastic events
Consequently, whenever the physics of recoil kinetics (mass, velocity, inertia, inelastic and elastic collisions) dominate and the influence of gravity is a negligible factor, the behavior of objects remains consistent even where gravity is relatively weak.

Elastic collision

elasticbouncescollision
Consequently, whenever the physics of recoil kinetics (mass, velocity, inertia, inelastic and elastic collisions) dominate and the influence of gravity is a negligible factor, the behavior of objects remains consistent even where gravity is relatively weak.