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Mass or Weight
In physics, mass is a measure of an object's inertia and the amount of matter in the object. Mass is determined by the object's acceleration and the force applied to it. According to Newton's second law of motion, the force (F) applied to an object is related to its mass (m) and acceleration (a) by F=ma. Weight is related to mass because it is the gravitational force on an object of mass. Weight (W) of an object is measured by W=mg, where g is the acceleration of gravity and m is the object's mass. The SI unit of mass is the kilogram (kg). The SI unit of force and weight is the Newton (N).

The object's weight can change depending on the gravity in its environment, while its mass will remain constant because the amount of matter (stuff) in it will not change. If the object would leave the Earth and go to the Moon, its weight would change, but its mass would remain constant. The Earth's acceleration of gravity (g) is 9.8 m/s², while the Moon's is only about 1.6 m/s². The object's 50 kg weight on the Earth will only show up as 8.27 kg on the Moon.
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Trivia  •  Fun Facts

Do you know?
Is mass the same as weight?
The relationship between mass and weight is frequently confused. It is often thought that mass means weight, but in physics, they are different properties.