WHO WANTS TO DIVE FOR TREASURE

[FISHEYE]

Tom got the right answer.

 

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[SEAHUNTER]

I don't know what things weigh underwater, but I will tell you that two or three guys can move a rock the size of my desk underwater and I seriously doubt we could do that above water. So maybe things weigh the same underwater but humans become stronger underwater

 

[Darren in NC]

The weight of an object is the same, whether it is in air or water.

I don't buy this, Fisheye. For instance, if this cannon indeed weighed 3000 lbs, Scott won't need 3001 lbs of lift to get the cannon up to the surface. Looking at the picture, I would guess the cannon takes up about 8 cubic feet, which means Scott could lift it with just over 2488 lbs of lift. Once it was at the surface, he would need more lift to get it out of the water.

 

[FISHEYE]

The lift associated with air spaces can be used to raise objects from the bottom. Since air weighs very little compared to the weight of the displaced water, it can be assumed that the lifting capacity is equal to the weight of the volume of water that is displaced by the air volume of the lifting device.

 

[riobravo]

brad you have the correct answer.
a boat floats because of its water displacement


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Pressure in Fluids

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Archimedes' Principle

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Fluids and Atmospheric Pressure

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Submerging an object suspended on a spring scale under water: Observation Experiment
Aim
The purpose of this experiment is to investigate how the force that a fluid exerts on an object submerged in it relates to the volume of the submerged part of the object.

Prior Knowledge
Some concept of conservation and incompressibility of fluid.
Free body diagrams.
Newton's laws as applied to equilibrium.
Description of the Experiment
Observe objects of various sizes and masses, suspended from a spring scale. Note the reading of the scale in air for each object. Note the level of the water in the beaker. Observe each object placed in the beaker. Record the new reading on the spring scale and the new level of the water in the beaker. Calculate the weight of water displaced by the object. Devise a rule relating the change in the reading of the scale and the weight of the displaced water.

Addtional Information
Make sure you use consistent units when doing the measurements and calculations. We suggest that you use SI units.

Frame rate:
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Size:
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File Size:
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Questions
First draw a free body diagram for any of the objects hanging from the spring scale before being placed in the water.


How do the force of the spring on the object and the weight of the object (the force ofthe Earth on the object) compare?


What does the spring scale measure?


Now consider the same object placed in the water.


What happened to the reading on the spring scale?


What happened to the force that the spring exerts on the object?


If you agree that the object is in equilibrium, where does the extra force come from to stop it accelerating downwards?


List all objects that interact with the submerged object. Draw a free body diagram for the object showing all the forces exerted on it.


Can you use the spring scale readings to determine the size of the extra force which appeared when the block was placed in the water?


Compare the size of this force with the weight of the displaced water


Devise a rule relating the amount of extra force and the weight of the displaced water.


Once you understand this, use the readings of all the different blocks to determine what variable the extra force depends on.


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watch video.......... http://paer.rutgers.edu/pt3/experiment.php?topicid=9&exptid=71

 

[ScubaDude]

I think Fisheye wanted to play with semantics. Realistically the weight of the object never changed, however it's "realistic" (how many lift bags do I need) weight due to buoyancy will decrease. I was trying to answer the question from a realistic point of view, not the Mr. Wizard semantics, trick question version.

Archimedes is one of the first things they teach you when you get your diving cert.
http://aquaholic.com/gasses/archem.htm

Lets not hijack Seahunter's thread. If anyone wants to discuss it further start a new thread in Diving & Underwater Salvage where it belongs.

 

[PDJ]

Seahunter,
In the 3rd and 4th pic I see a cannon ball that looks like the one I posted below. I assume you pulled that up -- What size is it?

 

[SEAHUNTER]

Hello PDJ
I agree with you that in the pictures they look similar, but the object in the picture is actually a coquina type rock that is about twelve inches in diameter and about three inches thick with the hole close to one side like a medallion. There were several natural stones or rocks with unusual formations in this hole. They may have been formed around or inside man made objects which deteriorated over time and left only the rock. Next time I will get better pictures of some of these. We intend to do a lot of exploring in this area.
Seahunter

 

[inletsurf]

The weight of an object is the same, whether it is in air or water.

However, if you move an object from sea level to a much higher elevation, it does in fact weigh just a little less, because you have moved it further away from earth's center of gravity.

Tom

You are correct about one thing: The mass of the object is the same regardless of it being in air or water, however the resultant force needed to float the object (this case weight) is dependent upon the density of the fluid surrounding the object. Known as Archimedes principle, the other posts on the board had it right. It is the weight of the object minus the weight of the water of the volume it displaces. And because water is nearly incompressible, this does not change significantly with depth.

Moving an object away from the earths center of gravity changes the effect of weight, but not mass. And it certainly doesnt' change it in any noticeable amount whether you are at sea level or on the top of a mountain.

 

[mad4wrecks]

The original question From Fisheye was :
If you have a cannon that weighs 3000lbs.what does it weigh underwater?

The correct answer was (and continues to be): The weight of an object is the same, whether it is in air or water.

Fisheye did not ask anything about lift or bouyancy and none of that is relevant to his simple question. Archimede's principle does not come into play here. You, like so many others, read WAY too much into what Fisheye was asking.

Maybe it was a trick question to test how well people read?

 

[inletsurf]

The original question From Fisheye was :
If you have a cannon that weighs 3000lbs.what does it weigh underwater?

The correct answer was (and continues to be): The weight of an object is the same, whether it is in air or water.

Fisheye did not ask anything about lift or bouyancy and none of that is relevant to his simple question. Archimede's principle does not come into play here. You, like so many others, read WAY too much into what Fisheye was asking.

Maybe it was a trick question to test how well people read?

See, because you are getting technical, your answer is wrong. Weight is a resultant force that changes with boundary conditions (gravitational field, buoyancy). A scale on a lift bag in neutral buoyancy with a cannon lifted off of the bottom would not read the same if the same scale was supporting the cannon in the air. Weight is relative. MASS is not. MASS is a unit of measure based on the number of atomic particles present in a body. The MASS never changes. Distinction between weight and mass is a big deal in physics or engineering. When asking for the mass of an object, you do not ask what is its weight. Maybe here you do, but in engineering applications you'd better be very descriptive of the answer you want in measurements, or you'll end up hurling a satellite into the surface of Mars at thousands of miles per hour. Which has happened.

Here is some more reading for you.

The weight force that we actually sense is not the downward force of gravity, but the normal force (an upward contact force) exerted by the surface we stand on, which opposes gravity and prevents us falling to the center of the Earth. This normal force, called the apparent weight, is the one that is measured by a spring scale.

For a body supported in a stationary position, the normal force balances the earth's gravitational force, and so apparent weight has the same magnitude as actual weight. (Technically, things are slightly more complicated. For example, an object immersed in water weighs less, according to a spring scale, than the same object in air; this is due to buoyancy, which opposes the weight force and therefore generates a smaller normal. These and other factors are explained further under apparent weight.)

If there is no contact with any surface to provide such an opposing force then there is no sensation of weight (no apparent weight). This happens in free-fall, as experienced by sky-divers (until they approach terminal velocity) and astronauts in orbit, who feel "weightless" even though their bodies are still subject to the force of gravity: they're just no longer resisting it. The experience of having no apparent weight is also known as microgravity.

The correct question to ask in the beginning would have been: "A cannon's mass is measured to be 3000 lbm in air at sea level. What is its mass 30 feet under salt/fresh water?" Then it would be the same.

BTW, when talking weight, the same cannon would take more lift bag volume to raise it out of fresh water than it would salt water, because salt water is denser.

Sorry to burn you at your own trick. But it is cool that you stayed at the Holiday Inn Express last night...

 

[Darren in NC]

You, like so many others, read WAY too much into what Fisheye was asking.

Us "so many others" apologize for hijacking Seahunter's thread and bantering over semantics and perspective. You don't have to be "right" on your own, bro. Both perspectives bring good thought to the so-called simple question and are "right" depending on what angle you're coming from. 8)

 

[jeff of pa]

Read Carefully

http://www.treasurenet.com/terms/

 

[inletsurf]

Hey Seahunter,

I'd like to dive with you one day. Let me know if you ever need another crew member. I am used to diving very crappy conditions very often. Thanks.

 

[SEAHUNTER]

Inletsurf
We would enjoy having you join us, where are you located. It is a little hard to schedule anything this time of year unless you are close due to the unpredictable weather.

Everybody else
Found all of your posts interesting and found out how little I know about physics, but something tells me with your help we will get the cannon off the bottom when the time is right. I assume the reason things feel so light underwater is because I am only lifting the rock, not the water it displaces.
Darn !! I was kinding of hoping I had extra powers underwater.

Seahunter

 

[diverlynn]

Scott,
Hopefully we will be back soon. I am sure the Polly L can bring it up.


Diverlynn

 

[wwwtimmcp]

nice cannon, what metal detectors do you use ?

 

[SEAHUNTER]

Hello wwwtimmcp
We use several different detectors. Garrett Seahunter, Fisher Pulse 8x, Aquapulse and Minelab Excalibur to name a few. But for the cannon just used our eyes.


Hi Lynn
We definitely look forward to seeing everyone aboard the Polly L again as soon as you can come back. I am sure the Polly L would have no problem with the cannon no matter what the physics are.

Seahunter

 

[SEAHUNTER]

Well we have not been able to dive for a while, but I came across a interesting picture from back in September that I thought you all might like.

 

[WhiteHunter]

looks fun to explore wish i was there