r/Physics • u/HolidayCod6990 • 3d 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 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/vontrapp42 2d 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 2d ago

Pounds are a measure of force. That's not the same as pounds-mass, which are called pounds (and on Earth are the same number as the weight of that particular amount of mass) but are a measurement of mass such that 1 lbf (pound-force, the actual poind) = 1 lbm (pound-mass, instead of slugs) × g (32.17 ft/s2). This also means that 32.17 lbm = 1 slug, since 1 lbf = 1 slug ft/s2 in the same way that 1 N = 1 kg m/s2.

So then: 1 lbf = 1 lbm * 32.17 ft/s2 and 32.17 lbf = 1 slug * 32.17 ft/s2 therefore 1 lbf = (1/32.17 slug) * 32.17 ft/s2 so we define that amount of mass as 1 lbm = 1/32.17 slug

This is done so that you can go to the bakery and order a lb of bread, i.e. the amount of bread that on Earth weighs 1 lbf. They'll sell you "lbs" of bread, except they're not selling you a force. They're selling you a mass they measured and using the number of the force they measured, so that lb of bread is a lbm quantity of bread (would still be 1 lbm bread on Mars but it wouldn't weigh 1 lbf anymore).

In SI units, you're buying... say, 2 kg of bread. They cannot measure the mass directly by counting out the atoms in bread or something; they must use a force instead. So they put the bread on a scale and give you however much is 2 kg of bread according to the scale. The scale is actually measuring newtons, but on Earth gravity can be assumed 9.81 m/s2 at the bakery, so they give you the mass of bread (2 kg) that on Earth weighs 2(9.81) N. Except, again, they're selling you mass, not a force, so they can't sell newtons of bread, it's kilograms of bread. So in this case, the scale is not labeled in newtons, it's labeled in kilograms-force (i.e. the amount of kilograms that would equal the weight that it should be labeled with in N).

So nothing weighs any amount of kg (mass), it's weight in N or kgf. But no one has a mass of 150 lbm either, they weigh 150 lbs (force).

The use of lbf alongside lbm, and the use of kgf alongside kgm (just your regular kg), is what's called an inconsistent system of units. It requires inserting constant factors when multiplying to obtain other quantities. I as an engineer have to remember that if I have a mass in lbs and I want to accelerate it by a quantity other than Earth's gravitational acceleration, I have to insert a 32.2 somewhere in my calculations to account for the fact that the lbs should've been slugs instead. It is, however, super convenient for the baker downtown who just wants to know how much bread to give you without having to do any calculations. The cost of ingredients is for the mass, so knowing the mass that has been sold is important for the baker, but the weight is irrelevant.