What is the difference between mass and weight?

In everyday conversations, and even on our website, we treat the terms mass and weight like synonyms. But scientifically speaking, the two are related but very separate. Body mass is an intrinsic property of this thing. It is the amount of matter in it. Your mass doesn’t change if you’re standing on Earth, on the Moon, or halfway to the Andromeda Galaxy. But your weight will change.

Weight is a force, specifically, the force acting on the substance of an object by a gravitational field. So the old trick question asked of children “What weighs more: a kilo (or pound) of feathers or a kilo (or pound) of lead?” Requires an important addition. Are these two samples in the same gravitational field? They have the same amount of matter. But a kilogram of feathers on Jupiter weighs more than one kilogram of lead on Earth.

Mass is important

The mass itself is a very complex matter when it comes to a full understanding. It arises from the interaction of fundamental particles with the Higgs field – heavy particles have stronger interactions. But there are some special properties of mass that can’t be easily explained.

The first of these is the principle of equivalence. Without involving particles, it is possible to measure mass in two ways: gravitational mass from weight as well as inertial mass due to the body’s properties to resist motion. These two in the universe are equivalent and this is also known as the universality of free fall.

Legend has it that Galileo showed this to be the case by dropping two cannonballs of different masses from the Leaning Tower of Pisa. There is footage of Apollo 15 commander David Scott experimenting with a feather and hammer on the moon. Recently, the equivalence principle has been tested with incredible rigor.

So while we generally talk about mass as something we have a super clear direct understanding of, when it comes to the science of it, there’s a lot of work to be done.

Floating does not mean weightless

And it seems like for the concept of weight, almost the opposite is true. There are things that might confuse us at first but are actually quite 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 balloon filled with helium. Scale does not record it. But the balloon, the gas inside it, and the string have mass. And we’ve learned that here on Earth, that means weight. But due to its low density, the balloon floats away. So where did the weight go? The answer is everywhere.

If this baffles you, don’t worry, you’re not alone.

The thing we are exploring is Archimedes’ principle: The body in the fluid experiences an upward impulse equal to the weight of the displaced fluid. This is the concept of buoyancy. The upward surge of fluid displacement balances or overcompensates for weight so that the balloon floats away or the ship does not sink.

Personally, the balloon example is still a bit confusing. But thinking about an airplane brings that back to reality. Your mind may deny that a helium balloon has weight even as it soars through the sky, but an airplane is one big metallic beast. It has weight on the tarmac and it will have weight when you fly. But you can’t measure it. This is because the weight of the aircraft is applied to the atmosphere, so it spreads across the planet.

So the atmosphere is a little heavier since we invented flying. How heavy is it? Based on the average aircraft mass (including paint) and the average number of flies in any given second, it is approximately the weight of two fruit flies per square metre. But, as we learned, this is not its mass.

All “illustrative” articles were validated by fact-checkers at the time of publication. Text, images, and links can be edited, removed, or added to at a later time to keep the information up-to-date.


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