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Our sand waterfall Aquarium II

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2PK Super Natural Peace River Sand – 5 Lbs (Catalog Category: Aquarium / Gravel Sand-freshwater)
Super Natural Peace River Sand – 5 Lbs. CARIB SEA – SUPER NATURAL PEACE RIVER SAND 5 LBSCARIB SEA SUPER NATURAL PEACE RIVER SAND …

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Desert Sand/Cactus Aquarium Background 12" x 24"

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Question by ScottVR: How much does this man in a box weigh? This helium baloon? (Lobsters, birds, bees, chimera, oh my!)?
Thinking of the weight of an aquarium full of water and lobsters got me wondering. At first I had this thought, “in water, a ten-pound lobster would only weigh…” but I didn’t even complete that silly thought before realizing that things only *feel* lighter in water due to buoyancy and feeling embarassed having even partially wondered about that. But the image of fish floating in water in an aquarium on a scale reminded me of a question I had as a child…

If a 200lb man stands in a 10′x10′x10′ box (sitting atop a scale) that weighs 100lbs, the combined weight is 300lbs. If he hangs from the top of the box, obviously the scale still reads 300lbs. If the box was filled with water (like in our initial aquarium scenario), the combined weight would be 200lbs + the weight of 1000 cu ft of water (some 62000 pounds it seems, though this number is irrelevant to the question) + the 100lbs of the box, regardless of his position in the box/aquarium (standing on the bottom, floating, etc.)

So, the main question is, if this man jumps – or better, since he can apparently survive in a sealed box full of water, let’s imagine he can levitate, or fly, hummingbird/bumblebee style by flapping his arms perhaps since that wouldn’t involve defying gravity. What is the weight measured by the scale? Why?

Thinking of this as a child, I remember jumping on my mother’s bathroom scale repeatedly only to find that the force exerted by my jumping and landing would cause the needle to bounce wildly and I imagined the same thing would happen if I was inside a crate that was on top of the scale, leading me to think that if I could hover in the sealed crate, only the weight of the crate would register.

However, if it was water, whether I was in contact with the box or not, the weight reading on the scale would stay the same, presumably because I was in contact with the water, which was in contact with the box. So then of course I thought about the weight of air, which I decided wouldn’t be measured because however scales work, they’re already taking atmospheric pressure into account by design, once tared to zero, which even now, just as when i was a child, gets me wondering of the weight of a helium balloon.

Helium, being “lighter than air”, the balloon being able to float in our atmosphere, but surely the weight of the balloon if it could be measured by the scale would be the weight of the deflated balloon plus the weight of however many cm^3 of helium, which causes me to wonder one more thing and that is, assuming the weight of our atmosphere effects the reading of the man in the box on the scale, how does the answer to the weight of our hypothetical hovering man change if the atmosphere in the crate were a vacuum.

The fact that I can’t imagine a way for my arm-flapping man to fly in the vacuum actually is what makes me think that as silly as asking the weight of the man-in-the-box seems, that it’s a valid question – at least for someone missing some principle he’d have learned if he didn’t drop Physics way back when – and that the answer may lie in the system’s measurement ability, rather than any actual changing weight of the system.

Someone who has made it this far in my rambling question, please clue me in. Or at least, point me at what concepts I should google to learn to answer this/these myself. Thanks!
sorry, I know it’s long enough as is, but I just re-read and if it isn’t clear after digressing about the box being filled with water, when talking about him flapping his arms, I meant to be clear that I meant this to be the “air-filled” version of the box again.
Jeff – I like your answer and since you like the question and have given a sensible answer, I’d like to extend the question a tad bit with some thoughts I left out originally just because it was already so long.

I made our man fly with wing-flapping, you mentioned a rocket engine in the vacuum – there is one other angle that seems even more worth mentioning now with your feedback. What if the man had jet engines or propellers, as in a plane. It is my understanding that in this situation we’d only have gravity exerting downward force, with thrust and drag in front and in back of our flying an, and the shape of his fixed-winged arms creating lift as long as he was in forward motion. I suspected the downward force that you explained nicely, which is why I mentioned my jumping experiments as a child. If you read this further, do you have any thoughts on the weight now that our man is flying in circles in the box? Thanks!
A-ha! Your answer caused me to elaborate with my question of the man now being an airplane which made me think that put that way – “an airplane flying in a giant box” – was likely to be addressed somewhere that google could find (a man jumping inside a box is seemingly absent from my queries) and voila! I found this:

http://www.physicsforums.com/archive/index.php/t-170218.html

So thanks again, Jeff. It seems that the forces of thrust and drag would also eventually “make their way down the jar” to the scale, which was indeed intuitive after your answer but I wanted to ask anyway.

Now, this being the first question I’ve asked on Yahoo! ANSWERS, darned if I can see a way to accept your answer. Hrmm..
– And just after pasting that link to that other forum, I see that you replied to my previous update. Thanks much! But I still don’t seem to have an option to accept your answer. I’ll figure it out eventually.

Best answer:

Answer by Jeff
I like your question a lot, and I think I have an answer for you. You really have to think about what it is that is making the man “fly” in the box. If he is jumping, then for the instant that he is in freefall, the scale would only read the weight of the box, not the man. But once he hits the ground, the scale needle will “bounce way up” as you described–the box will appear to weigh a whole lot more for the instant that he is hitting the ground. The effect is such that the average weight of the man in the box is still the total weight of the man + box. For an instant it’s lighter, and for another instant it’s heavier–overall it’s the same average weight.

Now, let’s think of what it takes to get someone to fly in a box. If you have a bird or an airplane or a helicopter fly in the box, all of these operate by the principle of force and reaction force. Things (denser than air) that fly have to push air downward, which causes a downward “wind”. Just stand below a helicopter taking off and you’ll know what I mean. You’ll see the same wind under a hummingbird or bumblebee flying near the ground. And if you’re thinking that if the thing flies high enough that you don’t feel the wind anymore, you’re right, but the wind results in a slight increase in pressure far down below the flying thing. What this means for the helicopter in the box is that the wind from the helicopter blows onto the bottom of the box, creating pressure or a downward force, and reducing the pressure on the ceiling of the box. The net effect on the box is to push the box downward. In fact, the downward force averaged over time will be exactly equal to the weight of the flying thing. So if you are weighing the box, you will weight the force (weight) of the box + the force of the wind from the flying thing, which equals the weight of the flying thing, so the net weight is the same whether it’s flying or not.

Now you ask what if there is a vacuum in the box. Well, that’s a great question, but you need to answer: how will you get the thing to hover in a vacuum? If you use a rocket engine, then the exhaust of the engine will hit the bottom of the box, creating a force equal to the weight of the rocket. If you use magnets, then the magnetic force on the attracting magnet is equal to the weight of the object it’s attracting. So if the levitating magnets are attached to the box, then the box will feel the force of the levitated object. BUT… if you were to use magnets that are outside of the box to lift the object in the box, then the counterforce would be on objects outside of the box, and the box will now be lighter–you will only register the weight of the box–just like what you are wondering. But if that sounds really cool and mystical to you, it really isnt. Why? It’s the same idea as if you lifted the man out of the box with a crane that’s outside of the box. Once you lift the man with the crane, the box gets lighter, of course–no mystery there, because now the crane is supporting the counterforce to lift the object, not the box. You wouldn’t be surprised if your box got lighter if a crane lifted something out of it, so you shouldn’t be surprised if an external magnet lifted something out of your box.

Simply put, if the lifting mechanism comes from within the box, you will always feel the total weight of all the things in the box, no matter what you do. If the lifting counterforce comes from outside of the box, then you will not feel the weight anymore, but you intuitively know that anyway, don’t you?

Scott:
I suspect you’re getting at the idea that a plane’s jet or propellers act parallel to the ground, rather than “pushing” the plane up? But you mentioned the secret there: lift is generated by the “shape of the wings”. You’re getting at the bernoulli effect and that famous image of the wings with air flowing above and below it, creating low pressure above and generating lift. But here’s the catch–now it’s no longer the propeller or jet that’s creating lift, but the wings instead. So what? The lift around the wings is exhibited by a pressure difference, and a pressure difference in air causes downward air movement behind the plane. In the case of a plane flying in the air, the lift pressure of the wings causes a downward blowing wind caused by the wings and not by the jet engine. So if you’re standing below a plane that’s flying overhead, you’ll still either feel an increase in pressure, or a downward wind, which causes a downward pressure. There simply is no way to fly on earth without causing a downward force somewhere, and that force is transmitted through whatever it’s in. Even if he’s flying in circles, the wings cause that downward force. There’s just no way around it, though I commend you for trying to figure out a way around it!

You can eventually try to think of some really complicated situation that neither I nor anyone else can explain, but that won’t change the fact that you won’t be able to do it, because ultimately it’ll go against the conservation of momentum and conservation of energy laws. It’s like when people try to come up with perpetual motion machines–they eventually make such complicated plans that they think that they’ve found a loophole, but ultimately they’ve just forgotten a little tiny calculation somewhere.

Notice the old riddle in the link posted below. It’s similar to what you’re asking. The answer that was given is actually wrong. Marbledog actually got the correct answer that juggling stuff does not make you and the stuff lighter, despite what many folks may say.

The only thing you can do to get around this to your advantage is to delay the effect of the force a little. Like if I give you a really tall box with a big frog in it. If he’s jumping while I hand the box to you, then you won’t feel the weight of the box until the frog hits the ground. But if you’re smart, you’ll set the box down for the instant that the frog hits the ground, and when he springs back up again, you lift the box again. That way, you can carry the box any distance without actually feeling the weight of the frog. That’s because when you set the box down, the net impulse is on the earth, rather than onto you. The total average weight is the same, but you can time your lifting so that it is lightest while you carry it and heaviest while it’s on the ground. And if you have a box that’s tall enough and a frog that can jump high enough, you can carry the box as far as you want–as long as the frog is in freefall within the box. That does agree with Newton’s laws of motion and using that delay factor is your way around the box issue.

Scott: I think you have to wait 4 hours or so to accept a “best answer”

Know better? Leave your own answer in the comments!

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Aquatic Fundamentals 30/38/45 Gallon Upright Aquarium Stand
State-of-the-art aquarium stands & canopies Quality aquarium furniture with moisture-resistant, powder-coat finish Sturdy aquarium…

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AQUARIUM PLANT STAND METAL ORNATE DESIGN HOME OFFICE DECOR

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Quick view of H2shOw Lost world set-up in aquarium.

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Marina Decorative Gravel, 1-Pound, Orange
The Marina Gravel helps you create a wonderful Aquascape in your aquarium. The epoxy coating makes the gravel inert and prevents t…

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Aquarium 6.5" Height Plastic Water Plant Decor Red Green w Black Base

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The Aqueon Essence furniture is both economical and functional. Spacious, open shelving allows the consumer to store aquarium supplies or to adorn them with decorations to seamlessly blend into any room. Essence Aquarium Stands are built with heavy-duty fiber board covered by water-resistant laminate to last for generations.

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Aqueon 06531 Essence 24 Aquarium Stand, Black

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AQUARIUM PLANT STAND METAL ORNATE DESIGN HOME OFFICE DECOR

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Question by darkelf2110: Aquarium plants and CO2 question?
I’ve found recipes online to make “homemade CO2″ yet all the pictures the aquariums have only plants. I was wondering if anyone has tried it and if the mixture is fish safe.

Yeast, baking soda, sugar, and warm water.

Best answer:

Answer by DanielC
It’s perfectly safe as long as you use the right amounts so that it doesn’t create too much CO2, lots of people use it.

What do you think? Answer below!

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Vibran-Sea African Sword Silk-Style Aquarium Plant, Large 13-14 tall, Plum
Vibran-Sea African Sword hand dyed silk-style aquarium & terrarium plant, Large 13-14″ tall, Green…

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Stainless Aquatic Plant Aquarium Fish Tank Pond Curve Tweezer Cut New

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