Let's Make Robots!

Need some help with physics question (non robot)

The ten year old really got me stumped on this one...

 

 

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Dan M's picture

Ok, I'm too tired now to draw it up, but here is what is wrong with the hose analogy.

any container under water will sink or float because of pressure on the bottom or top. The hose has no "bottom" or "top" for upward or downward pressure to be applied. There is only pressure inward from all around the sides.

Therefore it is not a valid analogy, but something totally different.

TheBenjaneer's picture

Simple. You assume that the whole thing is under water; which is different. This isnt entirely under water which totally changes the mechanics. You can't compare a device at the bottom of the ocean with a device that is underneath a glass of water; or for that matter the proposed device. Also those under water mines floated up once and stayed up. and required air to be added. They werent perpetual :L

That being said i think i'll bow out of this discussion and leave y'all to it.

Dan M's picture

Exactly. As to the hose being underwater, I was being facetious because the hose idea is so totally different from the question at hand.

Then referring to my comments about the ocean; that was because someone mentioned things not floating because of "the weight of all that water" on them. I was pointing out that even things at the bottom of the ocean would still float upwards, even though they have "all that water" above them

 

As to the mines, that is not quite true about them not being perpetual.

Here is a news item from 4 August 2011, less that a year ago where they found one still there after 67 years !

http://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-14411288

(Includes an underwater photo of it still in place.) 

The problem is they deployed a LOT of these anywhere they thought enemy ships were liable to travel and then clean-up after the war did not get them all. I read that our side set over 300,000 mines, and of those there could be as many as 50,000 still down there somewhere. Notice that the article indicates this one was a 13 foot size (~4 meters) mine. That's a B-I-G explosion. It was meant to sink large warships. It probably has never detonated because it was set at a depth that only the large warships would hit it and small ships have passed over it without harm.

(I also read, that they did later blow this one up after moving it to a "safe" location away from the North Sea pipeline.)

TeleFox's picture

Agreed, if all the balls are under water, the net force will pull them up towards the surface, where the pressure is lower.

All these analogies just seem to confuse things as none of them exactly match the scenario posed in Phoenix's design.

An analogy is a great way to describe or explain how something works, but you can't really use one as proof of concept. If anyone has any other ideas that can be derived from real maths and physics I would love to hear them.

antonio.caciuc's picture

Your drawing perfectly illustrates what I was trying to say in my reply with the rubber hose loop. 

Now people will start discrediting your drawing because if you go out swimming in the ocean you wouldn't want to use an earthworm stuck through what looks like the cylindrical eyes of a blue-eyed robot :)

PS HTML needs a <sarcasm> tag.

TheBenjaneer's picture

Great Visualization! I wont bother then. I was about to make one showing the difference between a ball at the bottom of a tank of water and a ball underneath the container in this idea (surrounded largely by air). And show the difference between the forces but this is the simplest and cleanest way of explaining it.

Very nice simplification. 

petruisan's picture

I thought at a perpetual motion system also, it's like this: there is a wall that divides a recipient into 2 halfs: one with water and one with oil. The oil and the water are at the same height and close to the top of the recipient. Now if we make a hole into the bottom of the recipient, into the wall, and somehow we can suck a little bit of oil into the water half, the oil will go to the surface, being less dense than the water, and go back to the oil half, keeping on forever. Now this obviously won't work because in the end the pression will equalize and it will stop working.

The main problem with perpetual  machines is the friction, even between molecules, but if you want to check some other interesting ideas, here's a link: http://www.youtube.com/watch?v=287qd4uI7-E

antonio.caciuc's picture

...is that water will flow into the side with the oil when you make the hole into the dividing wall, not the other way around.

And this will happen until the pressure on the oil side equals the pressure on the water side.

Dan M's picture

Fantastico !  This is an absolutely GREAT link.  All of you who think perpetual motion cannot work should look at this video.

http://www.youtube.com/watch?v=287qd4uI7-E

 

Thank you Petruisan

 

As I said elsewhere, the key is that these devices are not without sustaining energy. They are powered by gravity along with other considerations like centrifugal force.... They would not work if they were floating in space.  The force of gravity propels them, and that is exactly what would make Chris's son's idea work. water displacement and all that boil down to gravitational effect.

I think that is all I need to say on this subject.  [Bows out, stage left]

MarkusB's picture

What this video not shows is that all those machines stop after a while ;)