In everyday conversation, and even on our site, we treat the terms mass and weight like synonyms. But scientifically, the two are related but very separate. The mass of an object is an intrinsic property of the said object. It is the amount of matter in it. Your mass doesn’t change if you are standing on Earth, you are on the Moon, or halfway to the Andromeda galaxy. But your weight will change.
Weight is a force, specifically, the force exerted on an object’s matter by a gravitational field. So the old trick question asked to children "what weighs more: a kilo (or a pound) of feathers or a kilo (or pound) of lead?" requires an important addendum. Are these two samples in the same gravitational field? They have the same amount of matter. But a kilo of feathers on Jupiter will weigh more than a kilo of lead on Earth.
Mass in itself is a very complex matter (pardon the pun) when it comes to a complete understanding. It emerges from the interaction of fundamental particles with the Higgs field – heavier particles have stronger interactions. But there are some peculiarities about mass that are not so easily explained.
First among them is the equivalence principle. Without getting particles involved, it is possible to measure mass in two ways: gravitational mass from weight but also inertial mass due to a body’s properties to resist a motion. In the universe these two are equivalent and this also knows as the universality of free fall.
Legend says that Galileo showed this to be the case by dropping two cannon balls of different masses from the Leaning Tower of Pisa. And there is footage of Apollo 15 commander David Scott doing an experiment with a feather and a hammer on the Moon. More recently, the equivalence principle has been tested to incredible precision.
So while we commonly talk about mass as something we have a super clear first-hand understanding of, when it comes to the science of it, there is a lot more work to do.
Floating doesn’t mean no weight
And it feels like for the concept of weight, the opposite is almost true. There are things that might confuse us at first but they are actually well-understood. Chief among them is the concept of weight in a floating object. You can’t measure it though.
Think, for example, of a helium-filled balloon. A scale wouldn’t register it. But the balloon, the gas inside it, and the string have a mass. And we have learned that here on Earth, it means a weight. But given its low density, the balloon floats away. So where did the weight go? The answer is everywhere.
If that puzzles you, do not worry, you are not alone.
The thing we are exploring is Archimedes' principle: A body in a fluid experiences an upthrust equal to the weight of the fluid displaced. This is the concept of buoyancy. The upthrust from the fluid displacement balances out or overcompensates for the weight so a balloon floats away or a ship doesn’t sink.
Personally, the balloon example remains a bit mystifying. But thinking of an airplane puts that back into reality. Your mind might reject that a helium balloon has a weight even as it rises up in the sky, but a plane is a big metal beast. It has a weight on the tarmac and it will have a weight when it flies. But you can’t measure it. And that’s because the weight of the plane is applied to the atmosphere, so it is spread out across the planet.
So the atmosphere is a little heavier since we invented flying. How heavy it is? Based on the average mass of a plane (paint included) and the average number flying any second, it is roughly the weight of two fruit flies per square meter. But, as we've learned, that's not its mass.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.