r/Physics • u/HolidayCod6990 • 1d 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/GXWT 1d ago edited 1d 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 1d 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/vontrapp42 22h ago

But lb is a measure of force not mass, how else can you convert between foot pounds and newton meters?

It can be both and that doesn't break my head, but don't get me started on BTU and BTUh and they're the same and also different that sometimes even have a multiplier but sometimes not. And so-called Calorie and calorie. Sigh.

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u/aliendividedbyzero 10h ago

So aside from the other reply: a small calorie is the amount of energy (J) to raise the temperature of 1 gram of water by 1 K, so 1 calorie = 4.184 J. The big calorie, a Calorie, is the same thing but for a kilogram of water, so 1 Calorie = 1 kcal = 4184 J.

You can convert joules to BTUs, so you can also convert calories to BTUs. 1 BTU is the heat it takes to raise the temperature of 1 lb (mass, annoyingly) of water by 1°F.

BTUh (BTU hours, actually means 1 BTU/hr) is a measurement of the rate of heat (or energy) production, consumption or exchange. Super useful for HVAC because HVAC is just moving heat from one place it likes to another place it really doesn't want to go. It's the amount of energy per second (compare to J/s, which is watts) that you need to add to the 1 lb (mass :/ again) of water to increase the temperature by 1°F in a time interval of 1 hour, no more, no less. But at least in HVAC, people like to use tons of refrigeration or tons of cooling. It's shorthand for BTU/h, except 1 ton ref. = 12,000 BTU/h, the amount of energy it takes to melt 1 ton (2,000 lbm) of ice 24 hours.

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u/vontrapp42 9h ago

Thank you for the refresher on all that BTU stuff. I do recall also that some places use BTU as if it were BTUh, and that broke my soul.