Mass, gravity and weight
People often confuse mass and weight.
But in physics they are very different quantities.
Mass
The massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of an object is the amount of matter or âstuffâ it contains.
The more matter an object contains, the greater its mass.
An elephant contains more matter than a mouse, so it has a greater mass.
Mass is measured in kilograms, kg.
A 100 kg object has a greater mass than a 5 kg object.
An object's mass stays the same wherever it is.
So, a 5 kg mass on Earth has a 5 kg mass on the Moon because the amount of matter stays the same.
Weight
Weight is the force of gravity acting on an object.
Weight is a force and so it is measured in newtons, N.
The force of gravity is not the same everywhere and so the weight of an object can change from one place to another.
For instance, on earth, the downward force of gravity on, or weight of, a 1 kg mass is 10 N.
The Moon has a smaller mass than the Earth and the pull of gravity is only \(\frac{1}{6}\) that on the earth.
In other words, a 1kg mass will always have a mass of 1kg but has a weight on the Moon of 1.6 N.
Key points
- The mass of an object is the amount of matter it contains.
- Mass is measured in kg.
- Mass is the same everywhere in the universe.
- Weight is the force of gravity acting on an object.
- Weight is a force and measured in N.
- The force of gravity is not the same everywhere in the universe and so the weight of an object is not the same everywhere.