While moving to Pluto would cause drastic “weight loss”…

…it won’t do a thing for your mass.

The science museum where I work (<3!) has a scale that displays how much you would weigh on various astronomical bodies. I was standing nearby when a family from the UK stepped on, and one of the mothers expressed mild dismay that the display didn’t show both pounds and kilograms. I just had to jump in.

The display would be very uninformative if it displayed kilograms, as the number would be the same for every single planet. While pounds are a unit of force (like the Newton), kilograms are a measure of mass, not weight.

Recall from this post that mass and weight aren’t the same thing; mass is a measure of how much stuff is in an object, while weight is a measure of how hard the planet that you’re on is pulling on that object. Since weight is a gravitational force, it depends on the masses of both objects involved and the distance between the objects (for planets, this distance is the planet’s radius). Different masses and radii cause an object’s weight to be different on different planets.

The amount of stuff in an object, its mass, is constant. We often use weight and mass interchangeably because a given amount of stuff (say, 1 kilogram) will always have the same gravitational force acting on it anywhere on Earth (about 2.2 pounds). However, that same kilogram on Jupiter will still have a mass of 1 kg but will weigh about 5.2 pounds.

So, in the name of scientific accuracy, we won’t be updating the “How much do you weigh on other planets?” scale to include kilograms. Maybe we could try using stones?


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