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Mass vs weight: what's the difference?

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Used in a sentence

University Course Reader · STEM

The probe's mass is 80 kilograms on Earth and on the Moon, but its weight on the Moon is a sixth of what it was.

The reader highlighted one word in a textbook passage. Clicked broke down the physics term “mass” into plain English:

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Overview

Mass is a measure of how much matter an object contains, and it stays the same wherever the object goes. Weight is the force gravity exerts on that mass, so it changes with where the object is. A 70 kilogram person has a mass of 70 kilograms on Earth and on the Moon. On Earth they weigh about 690 newtons; on the Moon, about 110. Bathroom scales report kilograms, so what people call weight in daily life is mass in physics.
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Overview

Mass is a measure of how much stuff you're made of, and it is the same everywhere. Weight is how hard gravity pulls on that stuff, so it depends on the planet doing the pulling. Mars pulls with about 40 percent of Earth's strength, so a person landing there weighs about 40 percent of what they did at home, every gram still present. Scales are labelled in kilograms, the unit of mass, which is how everyone learned to call mass weight. 😎

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Detail

Mass is a measure of how much matter an object contains, and weight is the force gravity exerts on that mass. Mass does not change with location: a 70 kilogram person is 70 kilograms on Earth, on the Moon and anywhere else. Weight does change, because it depends on how hard the local gravity pulls. Earth's gravity pulls each kilogram with about 9.8 newtons, so 70 kilograms weighs about 690 newtons here. The Moon pulls each kilogram with about 1.6 newtons, so the same person weighs about 110 newtons there, and their mass has not changed. The two get confused because bathroom scales show kilograms. A scale measures the push on its surface, which is a weight, then divides by Earth's 9.8 and displays the answer as kilograms. Carry the same scale to the Moon and it reads about 12 kilograms for the same person, because the push is a sixth as strong and the scale still divides by 9.8. Physics keeps the two words apart because only one of them describes the object itself. A person who diets loses mass, and their weight falls with it. A person who moves to the Moon loses weight only, and every kilogram of them is still there.
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Detail

Mass is a measure of how much matter is in you, and it is the same everywhere you go. Weight is how hard the planet you are standing on pulls on that matter, so it changes when the planet changes. Earth pulls every kilogram of you with about 9.8 newtons. Mars pulls every kilogram with about 3.7 newtons. So an 80 kilogram person is 80 kilograms in both places, and weighs about 780 newtons on Earth and about 300 on Mars. Nothing about the person changed. The pull did. The kilograms are the person's; the newtons are the planet's. Mass also shows up where there is no pull at all. Float a fridge in the space station and it weighs nothing, but shove it and it resists just as it did in your kitchen, because getting matter moving takes effort whether or not anything is pulling on it. That is mass doing its job with gravity switched off. Weight needs a planet; mass needs nothing. So if it is weight you want to lose, a rocket does it faster than a salad. 😎

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Analogy

Ten apples cost more in an airport shop than in a village market, and they are the same ten apples in both. The apples play the part of mass: the amount you carry in, unchanged by where you take it. The price per apple plays the part of gravity, because each shop sets its own, and the bill plays the part of weight: the amount times the local rate. Gravity is a rate of the same kind, about 9.8 newtons for every kilogram on Earth and about 1.6 on the Moon, so the same person arrives at both with the same kilograms and gets a different bill. People muddle the two because nearly everyone shops in one place, so the amount bought and the bill paid feel like one fact. The first time you pay somewhere else, they come apart.
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Analogy

Mass is a measure of how much of something there is, and weight is how hard the local pull acts on it. Picture carrying a toddler out of somewhere. The child stands for mass: same kid, same 12 kilograms, every time. The pull that place has on them stands for gravity, and the effort it takes to get them out the door stands for weight. Out of the doctor's waiting room, almost no effort. Out of the gift shop, real effort. Off the sofa halfway through Bluey, two adults and a negotiation. The child never changed; the pull that place has on them did, and the effort followed. Swap the sofa for a planet and that effort is your weight: the same kid, times whatever pull you happen to be standing in. 😎
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Formal definition — The same term, explained the usual way

Mass is a measure of the quantity of matter in a body, expressed in kilograms, and is a property of the body itself that does not vary with location. Weight is the gravitational force acting on that mass, expressed in newtons, and equals mass multiplied by the local gravitational field strength, approximately 9.8 newtons per kilogram at the Earth's surface. Because gravitational field strength differs between locations, the weight of a body varies with its position while its mass remains constant. In everyday usage the word weight is commonly applied to what physics defines as mass, and instruments such as bathroom scales are calibrated to display mass in kilograms on the assumption of terrestrial gravity.

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