Weight and mass are not the same. In science these words have special meanings.
Mass is a measure of the amount of matter (material) in an object and is measured in grams (g) or kilograms (kg).
Weight is a measure of the force of gravity pulling on a mass and is measured in newtons (N).
Under ordinary conditions the mass of an object can be said to be constant (the object’s mass will be the same at different locations in the universe).
The weight of an object is not constant, since the force of gravity varies from place to place (the object’s weight varies depending on its location in the universe).
The size of the Earth’s gravitational pull on an object depends on its mass. The Earth pulls a 1 kg mass with a force of 10 N and a 2 kg mass with a force of 20 N. Therefore, the strength of the Earth’s gravitational field is 10 N/kg.
The moon is smaller than the Earth and pulls objects towards it less strongly. The Moon’s gravitational field is only 1.6 N/kg.
A given object will have the same mass on the Earth and on the Moon, but its weight on the Moon will be about 16% of the weight as measured on the Earth. For example, a human with a mass of 50 kg on Earth will weigh 500 N on Earth. On the Moon, this person will still have a mass of 50 kg but will only weigh 80 N.
The density of an object is a measure of how much material is packed into a given space. An object’s density is equal to its mass divided by its volume (D = M / V) and the unit of density is g/cm3. The average density of the Earth is 5.5 g/cm3. This means that on average there will be 5.5 grams of matter in every centimeter cubed on Earth.
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