# The difference between mass and weight

We often use the words ‘mass’ and ‘weight’ interchangeably, but they mean quite different things. Your mass is the same no matter where you go in the universe; your weight, on the other hand, changes from place to place. Mass is measured in kilograms; even though we usually talk about weight in kilograms, strictly speaking it should be measured in newtons, the units of force.

Mass is a measurement of an object’s tendency to resist changing its state of motion, known as inertia. Left to its own devices, an object will stay put or move in a straight line – think of a puck on an air hockey table. Unless some force – like friction, or banging into a wall – acts on the puck, it will keep sliding on the same path forever. Mass is a measure of how much force it will take to change that path.

Mass depends on how much matter – atoms and so on – there is in an object; more mass means more inertia, as there is more to get moving. (This is Isaac Newton’s idea of mass, which underpins his famous laws of motion developed in the late 1600s. It is not quite accurate at extremely high speeds, where Albert Einstein’s 20th-century theory of special relativity is required, but it’s fine for most everyday situations.)

Weight, on the other hand, is a measure of the amount of downwards force that gravity exerts on an object. This force increases with the object’s mass: the more inertia it has, the harder gravity pulls. On Earth’s surface, the force of gravity is about 9.8 newtons per kilogram.

It’s only because we are used to dealing with conditions on the surface of Earth that we use often use ‘mass’ and ‘weight’ interchangeably. When we say someone weighs 70 kilos, we really mean their mass – their weight would be 70 * 9.8 = 686 newtons. (This force – 686 newtons – is actually what bathroom scales are measuring. They convert it into kilograms for convenience.)

On the surface of the Moon, where the force of gravity is weaker (about a sixth Earth’s gravity, or 1.6 newtons per kilogram), a 70-kilogram astronaut would weigh only 112 newtons. This is about what an 11.5-kg object would weigh on Earth.

Even on the International Space Station, where astronauts are weightless, they still have mass: they must still exert force to push themselves off the walls or pull themselves along, and the bigger they are the more force it will take.

So a trip to space might be a quick way to lose weight, but losing mass is another matter.

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