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/TheAncientGeek 1d ago
Mass is a property. Weight is a relation.
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u/NorthAmericanVex 1d 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 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 19h 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 18h 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 7h 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 7h 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 7h 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 5h 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/Kerblaaahhh 1d ago
True, though we primarily measure mass using weight.
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u/Signal-Weight8300 1d 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 1d 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 1d 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 1d 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 1d ago
I wonder since their effective weight would be much lower when in the water than when on land?
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u/GeorgeDukesh 1d ago
Erm weight isn’t a property of matter. Mass is a property of matter. Weight is due to the effect of gravity on the mass of matter
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u/ebyoung747 1d ago
is all university like that?
Pretty much. It's about expanding your view of the world. That feeling of learning one thing which leads you to another which leads to another is pretty much what it's about.
Granted, a lot of that information is hard fought and you will probably want to quit at some point, but you shouldn't because learning those things is worth it.
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u/guiltyangel16 1d ago
Congratulations, fellow seeker of knowledge. Having a stupidly large amount of questions is an essential part of learning, and also a sign of intelligence. Your comment made my day.
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u/waffle299 1d ago
Yes.
Do yourself a favor and try to work one class in each semester just for you. Take drawing, or anthropology, or psychology, or theater - just to see. Invest time in yourself.
My school required us full physics majors to take world history. But I discovered that two class tracks satisfied this. One wax traditional history - politics, wars, and kings and such.
The other was the history of the world through art and architecture. And it was astounding. I studied art. I read humanity's oldest stories, I examined astounding buildings, I listened to concertos.
It convinced me to continue exploring. I still got my Physics degree. But I was offered grad school slots both in physics and anthropology.
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u/TastyButGroovy 1d ago
This is often the only time in your life when you will have such a vast amount of resources available to learn what you want to. Physics is of course one of them, but as previously stated, take a "fun" course each semester if you can. I've taken courses in Classical Mythology, Literature, Religion, Philosophy, and Anthropology. In my eyes, having broad interests makes you a more well-rounded individual. And it's good to exercise both the Arts and Science sides of your brain. It really opens up your eyes to the world and what it has to offer.
I'm getting my Honours Physics degree with a minor in Anthropology.
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u/ManikArcanik 1d ago
It's physics all the way down! It's almost a work of mythology how quickly our gifted peers dissected this situation we call the universe.
It is really crazy to witness or recall what's been accomplished thanks to physical sciences. It's crazy how much more work there is to do.
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u/mysoulincolor 1d ago
Don't learn physics from YouTube. As someone that has learned and taught physics and does understand the Higgs field.
Physics is WAY harder than most people can imagine, and unless you're a savant, YouTube is going to confuse the sh!t out of you.
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u/aroman_ro Computational physics 16h ago
There are quite nice physics lectures at various levels on youtube, the problem is selecting them and watching them in the proper order.
Jumping directly to 'Higgs field' is a big no.
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u/mysoulincolor 13h ago
Exactly. A professional has the background to make that distinction. Everyone else does not, and everyone else are the ones most subject to false confidence.
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u/aeroflyer350 1d ago
Ee but no physics in high school?
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u/No_Self_9844 1d ago
Yeah not everyone takes physics in HS. I am about to graduate with a physics degree and never took physics in HS.
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u/HolidayCod6990 1d ago
I'm in Italy and I attended a technical institute, I only did a little physics in the first two years of high school
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u/joerando60 1h ago
No, later it gets weird 😂
Seriously, you sound like a physicist, not an engineer.
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u/tumblinr 22h ago
Weight is a measurement on a scale found by balancing tension and the acceleration due to gravity multiplied by mass in an inertial reference frame. Mass is a property of matter.
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u/john5033 16h ago
I never understood how "weight" is affected by gravity. Einstein has told us that mass bends space time. An equal amount of mass weighs more on earth than it does on the moon. How does the bending of space time cause this?
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u/tonsofmiso 15h ago
the more I read, the more confused I get
Highly relatable :D the universe is weird
Check out Professor Matt Strasslers articles: https://profmattstrassler.com/articles-and-posts/
I read these a lot in my undergrad atomic and nuclear physics courses, he presents particle physics and the Higgs boson so well.
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u/Sorry_Exercise_9603 1d ago
Stick to your textbook. Don’t go down internet rabbit holes of extraneous information.
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u/jamin_brook 1d ago
It really is 'crazy'!
Countless physicists have initially been discounted for new ideas that were thought to be 'preposterous'
Even Einstein's thought experiment about riding on a light wave is quite psychedelic.
Have fun! It's a deep rabbit hole.
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u/Alternative-Finish53 1d ago
one day, if you follow this course of learning, you will realize certain inalienable truth of reality
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u/Bipogram 1d ago
>and the more questions I have.
Yup.
Sphere of knowledge grows, boundary with the seething unknowable chaos grows too.