Topic: The weight of things.......... (Read 9471 times)

polaris · « **on:** January 02, 2009, 06:41:01 pm »

Dear All,

If a ship might have enough space to be able to accommodate another ship inside it (a tanker for example), and if the ship inside the 'mother vessel' is floating on water captured inside the other vessel (viz., the captured vessel can float in and out if needs be - but be sealed in if required), what is the weight of the whole? Since the vessel inside the 'mother vessel' is floating on water, I presume that displacement and weight is proportional? - the water was there in the first place, so this is a constant... it just has something floating on it. If the water was to be pumped out, the weight of the inside vessel then becomes part of the 'mother vessels' weight'. So, the question is, with say a 'mother vessel' weighing 20k tons: what is the weight of the vessel with the inside vessel floating, and the weight of the inside vessel not floating??? - will the 'mother vessel' weigh more with the inside vessel not floating within it? - the inside water weight is 'irrelevant' in that if the inside vessel can come and go as it pleases, the 'mother vessels' weight does not alter as this vessel has a 'constant' so to speak... but with the water pumped out the inside floating vessel then becomes a dead weight - the inside vessel only becomes a burden on the 'mother vessel' if the water supporting the former is pumped out! - but the 'mother vessel' will then float higher and have added buoyancy to support the inside vessel??? - in other words it will counter act... the inside water only becoming a weight when it's sealed off from the outside water and pumped out.

Regards, Bernard

Ghost in the shell · « **Reply #1 on:** January 02, 2009, 06:49:17 pm »

thats one for archemedes

polaris · « **Reply #2 on:** January 02, 2009, 06:57:15 pm »

Dear Ghost,

It was just something I was thinking about on/off during the day while doing other things. It will get some little grey cells working somewhere I'm sure! - but it is an interesting little matter!

Regards, Bernard

polaris · « **Reply #3 on:** January 02, 2009, 07:07:35 pm »

Dear All,

To further complicate this: if the inside vessel was to weigh, say, 150k tons gross and still float inside a vessel weighing 20k tons, in theory if the water was pumped out the whole lot would sink!!!

I am sure there is an easy answer to this... somewhere! Too much physics for my liking!

Regards, Bernard

das boot · « **Reply #4 on:** January 02, 2009, 09:19:51 pm »

Someone has been watching far too much James Bond....

Rich

toesupwa · « **Reply #5 on:** January 03, 2009, 12:36:36 am »

Quote from: polaris on January 02, 2009, 06:41:01 pm

If a ship might have enough space to be able to accommodate another ship inside it (a tanker for example), and if the ship inside the 'mother vessel' is floating on water captured inside the other vessel (viz., the captured vessel can float in and out if needs be - but be sealed in if required), what is the weight of the whole? Since the vessel inside the 'mother vessel' is floating on water, I presume that displacement and weight is proportional? - the water was there in the first place, so this is a constant... it just has something floating on it. If the water was to be pumped out, the weight of the inside vessel then becomes part of the 'mother vessels' weight'. So, the question is, with say a 'mother vessel' weighing 20k tons: what is the weight of the vessel with the inside vessel floating, and the weight of the inside vessel not floating??? - will the 'mother vessel' weigh more with the inside vessel not floating within it? - the inside water weight is 'irrelevant' in that if the inside vessel can come and go as it pleases, the 'mother vessels' weight does not alter as this vessel has a 'constant' so to speak... but with the water pumped out the inside floating vessel then becomes a dead weight - the inside vessel only becomes a burden on the 'mother vessel' if the water supporting the former is pumped out! - but the 'mother vessel' will then float higher and have added buoyancy to support the inside vessel??? - in other words it will counter act... the inside water only becoming a weight when it's sealed off from the outside water and pumped out.

Bernard... Ease off on the paint / epoxy / CA fumes if i were you..

polaris · « **Reply #6 on:** January 03, 2009, 10:06:44 am »

Dear Rich,

You are perceptive. I didn't watch the film this time round (but knew it was on), and it was indeed that film that started me thinking about this on/off quite a few years ago.

There is a double buoyancy factor: something of a specific weight/buoyancy floating on water, with something of a far greater weight floating within it. This is the bare bones of the equation. In theory, a vessel weighing 20k tons, capable of 'containing' a floating vessel of 150k tons, would mean the 20k ton vessel would only have to move it's own weight - viz. 20k tons.

If, however, one has a hull of a vessel, you place another ship within it, and place weight within that, and then make it float by putting water in the hull, the whole and total gross weight will increase proportionately. However, if the water level within is kept at a constant with the water the main hull is floating on, the vessel within becomes a zero weight (viz., the inside vessel/weight weighs nothing!).

Interesting isn't it! I have and claim copyright!!!

Regards, Bernard

malcolmfrary · « **Reply #7 on:** January 03, 2009, 10:56:19 am »

If the 20K vessel, complete with water, gains another vessel, and the water contained remains aboard, then the weight of the added vessel is gained. If the contained water has a connection to the outside world, the added vessel will displace its weight in water to the outside world, and its total displacement will be unchanged.
Assuming that the 20K vessel is a HUGE hollow box that is capable of holding, say, 200K of water without sinking, then adding a 150K vessel to the inside (assuming 150K is its total displacement) and allowing the excess water to escape will leave the total displacement unaffected.
The Stirling lift wheel works on this principle, it has two open chambers filled with water. A barge from the high canal floats into the top one, displacing its own weight in water back into the canal. The gate is sealed, the wheel turns and chamber, water and boat are lowered. at the same time the lower chamber, with the identical weight, is raised. Whatever the weight of cargo, the two chambers are always the same weight. The only way a chamber could be made heavier than its mate would be to drive an object into it that was heavier than the water it displaced, and that would not, by definition, be a boat.

Having seen the film, I wonder at a submarine probably capable of 25kt on the surface and with the ability to dive getting caught by a 20kt (being generous) bulk tanker almost incapable of maneuvering. How the bow doors remained attached is a mystery as well. I did like the fittings in the escape pod, though.

Reade Models · « **Reply #8 on:** January 03, 2009, 12:20:03 pm »

Archimedes Principle - for that is what is being discussed here, states in simple terms that a buoyant vessel will displace its own weight (mass) of water.

'Displaced' is probably most easily defined as "moved to somewhere else".

Thus a floating boat placed in a tank of water (or any other fluid for that matter, regardless of its specific gravity) that is full to the brim, will cause the tank to overflow. The weight (mass) of water, or other fluid, that overflows will be equal to the the weight of the boat.

Any object that does not float i.e. is sunken, displaces its own volume.

http://en.wikipedia.org/wiki/Buoyancy

Malc

Mankster · « **Reply #9 on:** January 03, 2009, 04:10:02 pm »

It isn't as complicated as it first seems. If the water within the "mothership" can freely enter and leave then it is akin to a moonpool so the mothership is essentiallly a tug, there is no change in displacement or weight of the mother ship. If water in the flooded part of the mothership can be pumped in and out then the mothership is a semisubmersible with a big ballast tank that can be filled with water, another ship or a combination there of. Displacement will change according to how much ballast (water, second ship) is taken on board.

polaris · « **Reply #10 on:** January 03, 2009, 06:54:37 pm »

Dear Mankster,

This was my conclusion, and we are both thinking on the same lines, but I thought the solution couldn't be that easy!!! One would think that the potential for fuel saving could be significant, since there is the possibility that if the engineering could be overcome a 20k ton shell with a small engine might be capable of moving a great weight that would otherwise need a massive engine - which most cargo/container ships have. I know this thinking is very much 'in brief', but the possibility is there all the same - indeed, I might go so far in saying that it will happen one day.

Interesting isn't it.........

Regards, Bernard

(Malc, thanks for your tech. Post).

Colin Bishop · « **Reply #11 on:** January 03, 2009, 06:57:35 pm »

Don't make the mistake of confusing weight with mass/inertia! The International Space Station may be weightless, but you still need a lot of force to move it!

Reade Models · « **Reply #12 on:** January 03, 2009, 07:16:01 pm »

Quote from: Colin Bishop on January 03, 2009, 06:57:35 pm

Don't make the mistake of confusing weight with mass/inertia!

I wasn't...

Malc

Colin Bishop · « **Reply #13 on:** January 03, 2009, 07:46:30 pm »

Just a general comment Malc!

Max Power · « **Reply #14 on:** January 03, 2009, 07:58:43 pm »

Polaris, you have obviously had too much time on your hands over the festive period! (A perhaps a like too much libation

What is important is surely not the wt of the larger (mother ship) but the maximum displacement it is capable of when fully laden. If this does not exceed the unladen weight (20T in your example) plus the weight of any cargo (ship carried inside it) it will obviously sink. While the interior hold of the mother ship is open to the ocean and the vessel inside it is obviously floating itself. I doubt very much if it is possible to build a ship big enough to contain a vessel of 150T without the larger vessel having a possible displacement well in excess of 150 + 20 T. In fact now that I think about it again I am certain of it! Your whole question is a teaser and a red herring and I have just spent ten minutes trying to answer it

polaris · « **Reply #15 on:** January 04, 2009, 06:48:13 pm »

Dear Max,

Wish what you said re time was true... it's always the time when I can catch up on the things that can't be done at other times - and I very very rarely touch alcohol these days (so both factors wrong in this instance!

It's always best to read all Topic Posts: you will notice Mankster get's very close to the principal/idea, and I quote again what he said to save you looking back - see end of this Post.

10 mins. thinking about it... that all... shame on you! I have been pondering it on/off for a year or two. It's an interesting theory that needs hard maths/physics - which is beyond me!

Regards, Bernard

Mankster's Post
It isn't as complicated as it first seems. If the water within the "mothership" can freely enter and leave then it is akin to a moonpool so the mothership is essentially a tug, there is no change in displacement or weight of the mother ship. If water in the flooded part of the mothership can be pumped in and out then the mothership is a semisubmersible with a big ballast tank that can be filled with water, another ship or a combination there of. Displacement will change according to how much ballast (water, second ship) is taken on board.

polaris · « **Reply #16 on:** January 05, 2009, 10:34:01 am »

Dear Colin,

I have wondered about this in the past from time to time. Whilst something in a vacuum still has a weight, without gravity it doesn't - because it is 'floating' - but it has it's own weight all the same - but no resistance in it's movement apart from it's own weight... but without gravity there is no weight! It mustn't be forgotten that the Space Station is not out of range of gravity, and is constantly doing it's best to get back home! - so, yes, it does have weight... proportionately more than if it was in gravity free deep space. Once something is given a good 'kick' in deep space, huge speeds are obviously achieved due to no resistance... the matter is more how to slow things down before hitting something in this instance! - near perpetual motion! The 'slingshot' principal of using a bodies gravity to boost a space vehicle was/is a superbly brilliant piece of engineering physics.

A space vehicle motor relies on thrust obviously... but it always makes me wonder what the motor pushes on in a vacuum to achieve movement! Obviously thrust alone of course. Lot's of physics that was sorted out quite a while ago... mind you, things are moving onto ion/particle drive... makes one wonder how far this tech. will reach one day! - ships maybe???!!!

What has all this to do with ships floating on external water in ships floating on water! Well, they said it was impossible to fly, that the motor vehicle wouldn't catch on, that it was impossible to travel faster than 30mph in a train as one would die from the speed, and that there would only be a market for a handful of computers!!!!!!!!! I have always believed in the philosophy of looking at how things can be done as opposed to why they can't!!! - the former being the essence of all invention..........

Anyway, break over, warmed up again, so back to work!

Regards, Bernard

Colin Bishop · « **Reply #17 on:** January 05, 2009, 11:15:19 am »

Bernard, rocket motors as used in space rely on ejecting gas at high velocity to give them thrust. The gases go one way, the motor goes the other in direct reaction. Ion drives chuck out ions to give the same effect. Both "burn" fuel so they don't last forever.

You can see the effects of low resistance very easily on Earth though. Say you have a boat weighing 4 tons or so sitting on a trolley. The chances are that no matter how hard you push you will not be able to overcome its inertia and move it. However, if the boat is alongside a quay in still water you can start pushing and in a minute or two the boat will begin to move slowly as the water offers little resistance. Be sure to jump on board before the gap becomes too great.

Roger in France · « **Reply #18 on:** January 05, 2009, 12:03:39 pm »

Colin's explanation/example is a good one as it also illustrates inertia. You may not be able to start an object moving because of its inertia but once it is moving you need significantly less force/power to maintain or increase movement.

With space travel it is not useful to think of "thrust" despite that term being popular. As Colin describes, it is a reaction to the engine emission.

Thinking about weight and mass, it helps to remember that anything which exists has mass but it has no weight unless acted upon by gravity. The weight of a floating object in water appears to decrease because gravity is acting downwards on the water. Which is why you need a relatively less amount of force to lift an object from a depth of water but much more force to lift it clear of the water.

Roger in France.

polaris · « **Reply #19 on:** January 05, 2009, 01:44:43 pm »

Dear Roger,

Re weight under/above water. Yet a one kg. lead weight underwater (whatever depth), weighs exactly the same as the same above water. One little additional equation in this though is the added weight of the lifting line attached to same... the deeper it is the more the gross will be (& visa versa).

I wonder how much the mean sea level would drop if all floating vessels were taken out of the water at the same time!

Regards, Bernard

dreadnought72 · « **Reply #20 on:** January 05, 2009, 02:44:08 pm »

Quote

I wonder how much the mean sea level would drop if all floating vessels were taken out of the water at the same time!

About the same distance as the width of a spider web strand.

This page tells me that (in 2005) 36.9% of the world's fleet were oil tankers, and that they displaced 960 million DWT. So I'll suggest that the total mass of all ships displaced by the oceans is around the 2.6 billion tonne mark.

The oceans cover 3.61*10¹⁴ square metres, so on average each square metre displaces 0.0072 kg.

Since 1kg is equal to 1mm of ocean per square metre, 0.0072 kg is seven thousandths of a millimetre.

Andy #1963#

polaris · « **Reply #21 on:** January 05, 2009, 03:14:36 pm »

Dear Andy,

Thankyou for a very interesting input/Post. Didn't expect to get such a thorough tech. reply as that!

Regards, Bernard

dreadnought72 · « **Reply #22 on:** January 05, 2009, 03:45:35 pm »

Not at all. Hopefully it beats listening to me droning on about the potential difference between inertial and gravitational mass, and how this ties in with Einstein's Equivalence Principle.

...Which I nearly did.

Andy #1963#

polaris · « **Reply #23 on:** January 05, 2009, 05:42:50 pm »

Dear Andy,

I know there are some flaws in my ship-within-ship idea, not least of which is what happens in a rough sea(!), but, as always, and with a bit of determination and 'where there is a will there is a way' thinking, there is a way around everything. About 40 miles away there is a long and deep railway cutting: when the rly. was proposed many so called experts said it was 'impossible'... needless to say David Davies took no notice and simply did it! - Brunel did such things numerous times: one of the best that springs to mind being the Maindenhead Bridge (one of the widest low arched brick bridges ever built - still is I think).

Drone away! What were you going to say re inertia etc.?

Regards, Bernard

malcolmfrary · « **Reply #24 on:** January 06, 2009, 09:51:02 am »

Quote

Re weight under/above water. Yet a one kg. lead weight underwater (whatever depth), weighs exactly the same as the same above water.

No. It weights whatever it weighed above the surface minus the weight of water displaced. It will have the same mass as on the surface for the purpose of working out inertia.
Dangle a lump of lead from a spring balance, take a reading, then dunk it and take another reading.
If lead is about 11 times the density of water, there will be a 10% drop in the measurement.

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Author Topic: The weight of things.......... (Read 9471 times)

das boot

Reade Models

Reade Models

Max Power

Roger in France