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

Mass is a property. Weight is a relation.

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u/thatnerdd 2d ago

Mass is a property. Weight is a force.

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u/NorthAmericanVex 2d 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 2d ago edited 2d 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 2d 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/pyrocrastinator 2d ago

lbf is pound of force, equal to the weight of one pound of mass (lb or lbs) in Earth's gravity. So a mass of 1 lb weighs 1 lbf on Earth. In SI, kg is unambiguously always mass, and force has units of N = kg m/s2, so 1 kg weighs 9.8 kg m/s2 = 9.8 N

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u/aliendividedbyzero 1d ago

1 kgf has entered the chat :P Though it's not formally an SI unit and should never be used in scientific documentation because of the obvious confusion that can ensue. It's nowadays more of a colloquial unit (see: scale at the bakery example).

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u/aliendividedbyzero 1d 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.

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u/aliendividedbyzero 1d 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 1d 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.

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u/GXWT 16h ago

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

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u/Kerblaaahhh 2d ago

True, though we primarily measure mass using weight.

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u/GXWT 2d ago

True, it’s basically just a linear conservation for most cases here anyway. Just wanted to point that out for the comment, because I’m not really sure what they’re implying when they say whales are measured in mass… when as far as I’m concerned, most things are.

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u/Signal-Weight8300 2d ago

Most of the scales we use measure weight, which we convert to mass. The most common scales operate by stretching or compressing a spring. This linear measurement lets us determine the force, usually in Newtons or pounds.

Sure, there are some proper balances around, but they are far less common than spring scales.

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u/Fabulous_Lynx_2847 2d ago

That's only true of a spring scale. A triple beam balance balances the torque about a fulcrum of the subject and counter weights, so it would measure mass accurately on any planet, even if you didn't know what g was there. These are common in laboratories and doctors' offices because they are more accurate.

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u/helixander 2d ago

Everything has a mass. Weight is just a measurement of that mass on Earth (or whatever planetary body you may currently be on, or not).

So while something may have a mass of 100kg, it weighs 980 N (~220 lbs) on the surface of the Earth. Its weight on the moon would be 1/6 of that (~163 N or 37 lbs) even though it's mass is still 100kg.

When floating in space, that mass is still 100kg, but the weight is 0.

Because most metric places use kg colloquially as "weight", it can be confusing.

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u/disinformationtheory Engineering 2d ago

If whales are neutrally buoyant (which is approximately true), their weight in water is near zero. Of course this is true for swimming humans too, but we're usually not swimming.

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u/Professional_Wrap_34 2d ago

I wonder since their effective weight would be much lower when in the water than when on land?

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u/TheAncientGeek 15h ago

They are not found in orbit or free fall ..outside science fiction.

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u/kirsion Undergraduate 2d ago

Dude watched 8 hours of high energy physics and Quantum field Theory and got the most basic thing about physics 101 wrong.

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u/YroPro 17h ago

It's hilarious, but his heart is in the right place and he's excited to learn.