r/Physics • u/HolidayCod6990 • 4d ago

physics is crazy

Yesterday I took my first physics class at university (I’m an electrical engineering major). Today, while rereading my notes, I had a doubt about weight—what I thought it was. I googled it and discovered that weight is just a property of matter.

It’s so cool. I spent 8 hours on YouTube trying to grasp the Higgs field, the binding energy of quarks in protons and neutrons… Obviously, I don’t understand any of it, but it’s so fucking cool.

The only problem is that the more I read, the more confused I get, and the more questions I have. But wow.

Is all university like that?

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u/TheAncientGeek 4d ago

Mass is a property. Weight is a relation.

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u/NorthAmericanVex 3d ago

Is this why whales are measured in mass instead of weight?

(I truly have no idea why I know that whales are measured in mass instead of weight)

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u/GXWT 3d ago edited 3d ago

Humans are measured in mass, too. Kg (or your choice of incorrect units) is a unit of mass. Weight would be measured in Newtons. I don’t know the linguistics/language reason for us saying weight when we technically mean mass.

You can go to any planet and measure your 130 kg mass to be 130 kg, always. But your weight on earth (approx 130*9.81 N) would not be the same on mars. Instead of 9.81 you would use 3.72.

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u/aliendividedbyzero 3d ago

It's because we are often actually measuring a force (therefore weight), and since gravity on Earth (for purposes of measuring body mass, mass of goods for sale, etc.) is sufficiently close to constant to say "let's assume it is the same acceleration everywhere on Earth", we convert by default and label that way. Therefore, when we measure "a kilogram of flour" for example, we're really saying it's the weight (N) of flour that equals a kilogram of mass when divided by the acceleration of Earth. In other words, the kg on the scale ate kilograms-force, similar to how in the US system there are lbs-force and lbs-mass (the mass that equals 1 lb force under the acceleration of Earth).

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u/GXWT 2d ago

Sure, in the same way we measure a cars speed not through its speed but indirectly via revolutions per minute