Question about the weight of things deep in the ocean

[Arizona_928]

can't plug one into the other when they do not correlate.... different concepts.

I've seen a whole 700 chem class struggle with that concept last semester. which reminds me of the arguments here.

[javadog]

Don’t bring up chemistry, next we will be talking about moles…

[flatbutt]

Well then if we're discussing gravity we need to start talking about one of my favs the meter/second/second...weeeee! Oh oh how about Newtons kg-2 m2?

[island911]

Originally Posted by javadog

Actually, it weighs more when closer to the center of the earth.

If in space, then sure. But only because the the masses are coming closer. But as soon as there is building mass on the side opposite the center... well, consider how the moon and sun's gravitational fields distorts the earth. https://www.thoughtco.com/land-tides-or-earth-tides-1435299

Or imagine a planet of 100% water, with the block of steel at the center. What would be the big gravitational force on that block of steel, and which way would this gravitational field accelerate that block?

Originally Posted by javadog

That weight doesn’t change, the buoyant force just opposes it.

Smart ass! Of course saying that weight doesn't change leaves ambiguity about the weight measured. I mean, if you jump on a trampoline, are you not weightless in the air? There are no forces holding you in the air. You posses both mass and acceleration yet for a moment you are weightless - just as you would be in space.

Back to the ocean thing... If, as an engineering exercise, the questions arose of, how much force would be needed to suspend a 100_lb ingot of Aluminum in a freshwater lake?
would you answer 100_lbs ? Yeah, you probably would. -LOL

But would you show your work...
SG_Al = 2.6,
Suspension Force = 100-(100/2.6)= 61.54_lbf
Buoyancy Force = 100/2.6 = 38.46_lbf
Total Force = Buoyancy Force + Suspension Force = 100_lbf

Or would you leave those details of the answer hanging until your teacher marked "100_lbf" wrong? Not that I would ever do anything like that.

[javadog]

Pop quiz.

What’s the value of gravity of the center of the earth?

[island911]

Are we assuming a static spherically symmetric perfect density?

And/or do you mean center of mass?

[javadog]

No, I mean the gravitational constant at the center of the earth.

Assume the earth has an even distribution of mass, or just don’t worry about the rounding error.

[island911]

Gravitational constant is the point value, not to be confused with the small g of gravity at that point.

[island911]

...point value of the total mass.

Used for planetary interaction calcs and such.

[javadog]

OK, smart Alec, give me the answer for both big G and little g.

[island911]

Ok, the G is earth-sized massive whereas the g is zero.

If we had a tunnel thru the center you can imagine that you would fall to the center, with diminishing g as the center is approached and Coriolis kicking your ass the whole way down and back the other side a ways.

[dw1]

Sorry, but my first thought was: "Ok, a thread started by someone who didn't pay attention in science class and missed the explanation of weight versus mass..."

[Pazuzu]

Originally Posted by javadog

We were talking about physics here. In physics, weight is defined using two inputs, mass and gravity. That’s it.

I would say weight is one of the most misunderstood terms, as most people equate it with mass.

I would argue with you, and tossing terms like "misunderstood" is a dangerous thing with physics.

If you accept the equivalency principle of ma=F=mg, then "weight" is the total sum of forces on an object to keep it un-accelerated in the local coordinate system. That would mean that the buoyancy force is absolutely part of it, and the weight of an object within water is less than the weight of that object outside of water...in fact, a floating object has zero weight.

[masraum]

Originally Posted by javadog

We were talking about physics here. In physics, weight is defined using two inputs, mass and gravity. That’s it.

Buoyancy is another force, altogether.

Whatever’s in the dictionary that is in customary usage by the masses is somewhat irrelevant because very few of them can pass a basics physics exam. I would say weight is one of the most misunderstood terms, as most people equate it with mass.

I pulled into Nazareth, feeling ‘bout half past dead…

The OP never mentioned physics. There was no stipulation, hence the long discussion.

I think you ASSUmed physics was the only way to weigh in on this question.

[flatbutt]

Originally Posted by masraum

the only way to weigh in on this question.

clever bugger you are!

[javadog]

Really? What other branch of science might deal with this question?

[island911]

How about Ocean science?

I've done some design work for deep (miles down) submersibles. Controlling neutral buoyancy is kind of a big deal.

[island911]

Check out syntactic foams which resist this...

[Rusty Heap]

Originally Posted by javadog

OK, smart Alec, give me the answer for both big G and little g.

Seems like someone couldn't find her G Spot.

[porsche930dude]

Id say it only seems like things might weigh less because they fall to the bottom slowly. Both because there is still air in them and the resistence of the water