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# A drop in the ocean!

## or a rock as the case may be.

#### Mike12345

3:27 pm on May 21, 2003 (gmt 0)

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joined:Nov 28, 2002
posts:317

I was just wondering if anyone can help me here.

If I were to place a wooden board in a swimming pool 20 metres long and 10 metres wide, and then placed a rock weighing 10kg on top of the board. Would it submerge to the same depth if i were to take the same board and the same rock and place them in the sea?

It may seem like a bizarre question, but its quite important to me.

Any help greatly appreciated.

#### buckworks

3:36 pm on May 21, 2003 (gmt 0)

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posts:5837

Based on the details given we can only say "it depends". If the rock/board combo were light enough to float, it would "ride higher" in the sea because the salt content of the water makes it more bouyant. If the combo were heavy enough to sink to the bottom, it would sink deeper in the sea because the sea is deeper!

#### Macguru

3:38 pm on May 21, 2003 (gmt 0)

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posts:3300

No, because sea water has a higher density.

[edited by: Macguru at 3:39 pm (utc) on May 21, 2003]

#### jdMorgan

3:38 pm on May 21, 2003 (gmt 0)

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No, because the specific gravity of seawater is higher than that of a freshwater pool.
The rock and board would not submerge as deeply in saltwater.
This assumes that the board and rock are stable, and that the combination of the two can float in seawater, i.e., that it's not a "trick question."

Jim

#### Marketing Guy

3:39 pm on May 21, 2003 (gmt 0)

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posts:1958

Plus there are additional forces in action in the sea (tides, currents) that would effect the position of the board.

Scott

#### edit_g

3:39 pm on May 21, 2003 (gmt 0)

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posts:1590

Are we making any assumptions here?

How big is the log and how much does it weigh?

Calm water displacement would be different to choppy sea water displacement (I guess).

Is it tied to the log?

Cool question - I haven't a clue how to solve it... ;)

#### Paul in South Africa

3:39 pm on May 21, 2003 (gmt 0)

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posts:180

The scientific answer (if I can remember university physics properly, it was a long time ago) is that it will submerge slightly more in the swimming pool that it will in the sea. Practically however, the difference will be so slight as to be insignificant. The reason for this is that seawater is slightly more dense that fresh water and thus a 10kg rock placed on top of your board will displace slighly more fresh water than sea water.

#### trillianjedi

3:42 pm on May 21, 2003 (gmt 0)

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posts:7256

If it's a basic displacement question, then the answer is yes.

The mass of the water is different, but the object's mass is the same - it still displaces the same amount of water.

Anyway, if the question is about displacement in salt water, then no, it will be slightly different. Higher salt content = less displacement.

TJ

#### hayluke

3:52 pm on May 21, 2003 (gmt 0)

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posts:109

what about if the rock and board sank to then bottom of the pool, surely then it would sink further in the sea (as long as went you went far enough uot and the pool wasn't REALLY deep).

#### Mike12345

3:57 pm on May 21, 2003 (gmt 0)

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joined:Nov 28, 2002
posts:317

Thanks all, so the difference is only small?

Ill try to be a bit more specific now to calirfy it my head, im not good at stuff like this, physics or whatever you call it, robots are my game, although physics is involved, thats what you guys are for right ;)

Lets say the pool is full of sea water, and the salt content of that water is near as damm it the same (if thats possible?)

If the rock is now a can half full of air and half full of water, it will submerge in the pool to say... x metres but no further. Like a submarine?

And then i place it in the sea which is obviously a hell of a lot deeper, will still submerge to x metres or thereabouts or will it submerge to y metres? which is clearly too far.

Hope im bein clear enough here, thanks

#### trillianjedi

4:16 pm on May 21, 2003 (gmt 0)

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It will still submerge to "x" metres in the sea.

TJ

#### le_gber

4:18 pm on May 21, 2003 (gmt 0)

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posts:1555

Hi Mike12345, let see if I understood you what you want to know:

You want to know if one object will sink lower depending on the depth of the pool/sea?

I think that if you put a can half full of water (is it salty water or not ; ) it will only sink as low as the level of water in it (might be a bit above the water level because of the salt)

Damn I was never good in physics why did you have to ask that ;(!

Leo;)

#### trillianjedi

4:23 pm on May 21, 2003 (gmt 0)

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it will only sink as low as the level of water in it

Only if the mass of the container is zero.

TJ

#### Mike12345

4:26 pm on May 21, 2003 (gmt 0)

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joined:Nov 28, 2002
posts:317

Cheers guys. I think i understand now, your all life savers, i can get on with it now. :)

#### le_gber

4:29 pm on May 21, 2003 (gmt 0)

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posts:1555

Only if the mass of the container is zero

Yep sorry, I told you I wasn't good at physics ;)

Leo

#### tbear

6:06 pm on May 21, 2003 (gmt 0)

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joined:Feb 10, 2002
posts:937

Phew, seems this thread got picked up faster than any of those other threads...... LOL

So long as there is room for the log to sink, I doubt it will sink in a significantly different manner because of depth.

#### cornwall

7:42 pm on May 21, 2003 (gmt 0)

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joined:Sept 5, 2002
posts:1713

Attacking the problem from the pragmatic point of view, rather than the thoeretical, I remember doing life saving tests and opting for the tests in salt water because of the greater buoyancy.

If a human equates to your rock half full of air, the human certainly floats better in sea water. Your feet stay on the surface rather than sink!

#### Sinner_G

8:01 pm on May 21, 2003 (gmt 0)

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joined:June 27, 2002
posts:959

Why do I get the feeling you did someone's homework?

;)