Physics question

[August]

A buddy of mine claims that two ships located in sight of each other on a becalmed sea will always slowly drift towards each other until they collide.

Is this true or is he telling me a sea story?

[the_tyrant]

nope, because on a real sea and on a real ship
there are too many things influencing movement

[TLAM Strike]

No, the ships would need to be massive...

The force of attraction between small masses is negligible. As the mass increases, so does the force of attraction increases accordingly, in direct proportion to their product. Many, are prone to exaggerate this force. A person, not a physicist tried to assure that the frequently observed mutual attraction between the ships at sea was a result of gravitation. However, it is easily reckoned that the gravitation has no role in it. Two 25,000-ton battleships will attract each other with a force of only 400g, when they are 100 meters apart. This is naturally too little for the ships to move. The reason for this attraction is entirely different.

[tater]

In a fantasy perfect fluid (not a real body of water, that's for certain) in a thought experiment I'd wonder about surface tension when they were very close. Other than that, yeah, the gravitational attraction is so tiny that all the real life complications would make it noise (even becalmed waters have currents, fish, etc—the latter bumping into the boat would have more impact than gravitational attraction.).

Now if we were talking about spacecraft...

[August]

Tlam that refers to moving ships. This was about becalmed sailing ships. He says you can observe this phenomenon with two wooden matches in a bowl of water.

[Castout]

Perhaps this will help.



Or this



won't happen if they are not close enough from the start..maybe

[Aramike]

Originally Posted by August

A buddy of mine claims that two ships located in sight of each other on a becalmed sea will always slowly drift towards each other until they collide.

Is this true or is he telling me a sea story?

I'll go with sea story, because physically it makes no sense. If the sea were completely calm with no forces interacting with the vessels other than flotation, than the vessels would actually move only in accordance to the rotation of the earth. Clearly seas can never be completely calm and therefore there are always forces. Instead he must be suggesting that all forces would cause vessels, independant of said forces, to intersect, but that makes no sense because they could not move independant of said forces.

Finally, he would have to be implying that all the forces involved by themselves must intersect, and they would be completely impossible in all circumstances, because should all forces intersect at all places they must be completely equal and would therefore cancel one another out.

PS: By forces I mean, currents, wind, etc.

[Skybird]

If he means it as an illustration of principal gravitational attraction between two masses, then he is right in a way, but given the specific details of the example he'd choosen, other (environmental, global, astronomical) effects and variables add to the formula so that the virsible effect, I would expect, is so minimal that you can no longer observe it within a reasonable timeframe.

The example of two matches in a glass of water, if taken as a literal experiment indeed, I would see more as a demonstration of surface tension of water and what it can do, also maybe helped by the coriolis force.

General law of gravitation:

FG = G * ( M1 * M2) / r^2

F = resulting gravitation force
G = Gravitation constant
M1, M2 = two masses
r = distance between the masses' centre

The force of gravitation between two masses is directly proportional to the mass of the two objects. The force of gravitation is in an inverse squared relation to the distance between the two objects: you double the distance, but the force looses 75% in effect.

Both objects in this example'S setup, are object two two much more dominating gravitational forces that interfere with them: that of Earth itself, and the moon.

Summary: while true in principle, I would not expect to see a visible, gravitation-caused effect between two sailing ships that are in sight to each other, but some miles apart on the ocean: at least not within a reasonable timeframe for observation. the likelihood to notice the said effect, would fall rapidly with growing distance between both objects. I couöld imagine that the said effect just becomes evident when both ships (the centre of their masses, that is) are so close to each other that their outer hulls stand in their ways, crankle against each other, and prevent any further movement towards each other.

[JokerOfFate]

I'm confused

[Herr-Berbunch]

What if one, or both, had a hold full of magnets? Which makes me wonder, which country produces the most magnets for export, and how do they ship them successfully?

[tater]

There is a scale issue with matches in water, and what that shows is surface tension issues. In the ocean—even dead calm—the situation is not as idealized as in a cup of water.

Functionally, in the sea, any wind driving one ship directly towards another (in-detectible breeze, say) would push the two together. Any breeze not directly on a line between the two would push BOTH ships maintaining their separation. Move the breeze randomly, and you'll see that the breezes will then tend to push them both in the same direction OR will push them together (perhaps this is where this notion comes from?). Summed up, they will be pushed together since all other breezes have no effect on separation.

[August]

Here's how the subject came up.

We were watching this BBC miniseries about a voyage to Australia set in the early 1800's. I forget it's name but the guy who starred in Jurassic park was in it.

Anyways, there's a scene where their ship is becalmed in the doldrums and just as night is falling they spot another ship off in the distance, also becalmed. It being too far away identify they were forced to prepare for battle in case it was French since, according to the 1st officer "the ships would naturally drift together eventually unless the wind picked up".

Now I said that sounded like a load of bovine feces to me but my friend swore it was a real phenomenon. Since he has a navy background and I most certainly do not, I felt out of my element to argue my opinion and figured that i'd ask you guys being the only group of experts in the subject that I knew.

So thanks everyone for your replies. Unless this additional information changes anything i think i'm gonna tell him he is full of it next time I see him.

[FIREWALL]

Originally Posted by August

Here's how the subject came up.

We were watching this BBC miniseries about a voyage to Australia set in the early 1800's. I forget it's name but the guy who starred in Jurassic park was in it.

Anyways, there's a scene where their ship is becalmed in the doldrums and just as night is falling they spot another ship off in the distance, also becalmed. It being too far away identify they were forced to prepare for battle in case it was French since, according to the 1st officer "the ships would naturally drift together eventually unless the wind picked up".

Now I said that sounded like a load of bovine feces to me but my friend swore it was a real phenomenon. Since he has a navy background and I most certainly do not, I felt out of my element to argue my opinion and figured that i'd ask you guys being the only group of experts in the subject that I knew.

So thanks everyone for your replies. Unless this additional information changes anything i think i'm gonna tell him he is full of it next time I see him.

Better yet. If you really want to get even, tell him Skybirds spin on this.

[AVGWarhawk]

It would only be two ships passing in the night. No other influences.

[tater]

Originally Posted by August

Here's how the subject came up.

We were watching this BBC miniseries about a voyage to Australia set in the early 1800's. I forget it's name but the guy who starred in Jurassic park was in it.

Anyways, there's a scene where their ship is becalmed in the doldrums and just as night is falling they spot another ship off in the distance, also becalmed. It being too far away identify they were forced to prepare for battle in case it was French since, according to the 1st officer "the ships would naturally drift together eventually unless the wind picked up".

Now I said that sounded like a load of bovine feces to me but my friend swore it was a real phenomenon. Since he has a navy background and I most certainly do not, I felt out of my element to argue my opinion and figured that i'd ask you guys being the only group of experts in the subject that I knew.

So thanks everyone for your replies. Unless this additional information changes anything i think i'm gonna tell him he is full of it next time I see him.

Actually, it's not entirely implausible for the reason I suggested above WRT light winds assuming the ships are fairly close. The above scenario implies they are a decent fraction of a mile apart at minimum, or they'd be shooting anyway.

To be clear, imagine the two ships. There are almost imperceptible winds, randomly shifting through the points of the compass. On the off chance wind comes along a direct line between the ships, the wind hits the windward ship, pushing it towards the 2d ship. The air is disrupted by the windward ship, and so pushes the leeward ship less. Again, this requires they be close enough that the leeward ship is in turbulent flow (a wind shadow from the windward vessel).

Note that the same could also be true of subtle wave action. Wave hits the "windward" (in this case "waveward" ? ) ship, pushing it towards the 2d ship. Assuming the wave loses some small energy in the interaction, the 2d ship will have less energy delivered to it.

I still seriously doubt this in real life conditions, but it's interesting none the less.

I suppose as a sailor in that situation more than once there are 3 cases. No relative movement, relative movement apart, and relative movement drawing them together. Of the 3, only the last is memorable, because only the last is a navigation hazard. They forget the other two, and come up with myths about the latter.