OP
H2Andy
Contributor
simbrooks:just remember the Speigel Grove wont float due to it being full of water
et tu, simon?
Water as weight rather than lead?
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et tu, simon?
I see no knife, its august not march, FL not Rome, and i dont wear togas, do you?? But besides that, remember to keep an eye over your shoulder!H2Andy:et tu, simon?
Former FLL Now MSP Diver
Contributor
Here's two articles that should explain it all - both fairly simple, and one with some math.
Marc
Archimedes Principle and Buoyancy
Some objects, when placed in water, float, while others sink, and still others neither float nor sink. This is a function of buoyancy. We call objects that float, positively buoyant. Objects that sink are called negatively buoyant. We refer to object that neither float nor sink as neutrally buoyant.
The idea of buoyancy was summed up by Archimedes, a Greek mathematician, in what is known as Archimedes Principle: Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
From this principle, we can see that whether an object floats or sinks, is based on not only its weight, but also the amount of water it displaces. That is why a very heavy ocean liner can float. It displaces a large amount of water.
Archimedes principle works for any fluid, but as divers we are mainly concerned with two different fluids: fresh water, and salt water. We need to think of fresh water and salt water as two different fluids because equal volumes of fresh water and salt water do not weigh the same. For example, a cubic foot of fresh water weighs approximately 62.4 lbs, while a cubic foot of salt water weighs approximately 64 lbs. The extra weight is because of the dissolved minerals in salt water.
Let's take a moment and look at an object in water and Archimedes Principle. If you place a 1 cubic foot object that weighs 63 lbs into fresh water, the object is displacing 62.4 lbs of water, but weighs 63 lbs. This object will be negatively buoyant - it will sink. It is however being buoyed up with a force of 62.4 lbs, so if we weighed it in the water it would only weigh .6 lbs.
If we put the same object into salt water, it would still weigh 63 lbs, but would be buoyed up by a force of 64 lbs, and it would float. It would be positively buoyant in salt water. To make the object neutrally buoyant in salt water, we would have to add 1 lb of weight to the object without changing its size (without changing is displacement). Then it would weigh 64 lbs, and be buoyed up with a force of 64 lbs, thus being neutrally buoyant.
Let's expand on this and look at an example of putting these ideas to work in a real situation. Suppose you know that a boat had lost an anchor weighing 100 lbs. Measuring a comparable anchor, we find out that the anchor displaces 1/2 cubic feet of water. We will also assume that the anchor was lost in a fresh water lake. You do a dive and find the anchor and want to bring it to the surface but the only resource you have available are some 1 gallon milk jugs. How many would you need to tie on to the anchor to float it to the surface?
At this point we need to do a little simple math. We know that a cubic foot of fresh water weighs 62.4 lbs, so the anchor displacing 1/2 a cubic foot of water would be buoyed up with a force of 31.2 lbs. Let's round this to 31 lbs for simplicity. This means our anchor that weighs 100 lbs on land will weigh 100-31 or 69 lbs in the water. We now know we need enough 1 gallon milk jugs to generate 69 lbs of lift.
Perhaps you remember the old expression "A pint a pound the world around." This refers to the fact that a pint of water weighs about a pound. Since there are 8 pints in a gallon, we know a gallon of water must weigh about 8 lbs. Since we know a cubic foot of water weighs 62.4 lbs, this means there are about 8 gallons of water in a cubic foot. Let's put it together and solve our anchor problem.
If we need 69 pounds of lift, we divide 69 by 8 lbs per gallon to learn we need 8.625 gallons of water displacement to make the anchor neutrally buoyant. This means, we could fill 9-one gallon milk jugs with air to lift our anchor.
Let's try another. A 3 cubic foot object weighing 400 lbs is dropped into the ocean. How big of an air lift bag (in cubic feet) would you need to lift the object?
First we determine that a 3 cubic foot object in salt water would have 3x64 lbs of lift, or 192 lbs of buoyant force. If we subtract 192 from 400 we get 208 lbs. This means we need to generate 208 lbs of lift to make our object neutrally buoyant. We then divide 208 (the objects in water weight) by 64 (the weight of a cubic foot of sea water) to get 3.25 cubic feet of displacement is needed to make the object neutrally buoyant. Thus, we would need at least a 3.25 cubic foot air lift bag.
Archimedes Principal
Imagine you jump into a swimming pool and start paddling with your feet. If you stop paddling, you begin to sink, and if you sink you dive to a certain depth. If you have air in your lungs, and if you relase this air as you submerge, you will dive faster. These ideas are explained in terms of physics, and the big principal is called "Archimedes Principal." This concept basically says that the net force pushing you back up, counteracting your fall/dive is equal to the weight of the fluid displaced, the fluid in this case being water. Of course you can apply the same idea to a falling man in the earth atmosphere, the fluid in that case is the air.
The idea is really simple to understand, but some explaining needs to be done. Imagine you are holding a rock at some height off the ground. If you release that rock, it falls at some speed. Well, Isaac Newton once said that the net force that pulls this rock down is equal to the objects (the rocks) mass times how fast it is accelerating:
F = M * A
But it might not me intuitively obvious that there is a force pushing the rock back up, counteracting its fall. This force can be thought of as the combination of the force of friction (since the rock collides with air molecules) and the force of buoyancy. It is easy to understand friction. Just slide on the carpet and try to figure out why you get burnt, or why your basketball stops rolling on a horizontal surface without given some initial push, and you not pushing it. Well, if you imaging all the molecules on the edge of some surface colliding with your object, then you realize what friction is. Just try running through a bush, you will come to realize that it is not easy because the bush resists your movement. Now the concept of buoyancy is not as intuitively obvious. Archimedes principal essentially explains this force of buoyancy. Basically, when you are falling through some fluid, the force that is pushing you back up, the force that keeps you from falling, in addition to friction, is this force of buoyancy. Archimedes principal says that this force is (once again) equal to the weight of the displaced fluid:
Fb = r * g * V
Fb = force of buoyancy
r = Density of the fluid being displaced
g = gravitational acceleration constant
V = volume of object submerged
As the diagram shows, if you assume that the objects center of gravity is in the center, then the force of gravity acts in such a way as to pull the object down and the force of buoyancy acts to pull the object up.
Diving and Surfacing, Submarines When an object is floating, the two forces acting on it (assuming force of friction is negligible, reasonable since the object is not moving), force of gravity (Fg) and force of buoyancy (Fb) are equal in magnitude, opposite in sign, hence they cancel about the objects center of mass. On the other hand, when the object gravitational force is greater than Fb, the object sinks. Lastly, when the objects buoyant force is greater than its Fg, it rises. This idea is applied to submarines. Submarines are heavy pieces of steel with torpedoes. They sink and surface by using the laws of physics and by using pumps and tanks that hold compressed air. When these tanks are full of compressed air, the submarine rises in elevation. When these tanks are empty and in some cases full of water, the submarine dives.
It is essential to understand that a submarine is not just a heavy piece of steel, for inside the submarine there is living area. Thus a good fraction of the submarine is composed of air, and only a small fraction of the total submarine is composed of heavy steel. Since steel is really dense and heavy, one needs a lot of air (hence, one needs to displace a lot of water) to counteract this force of gravity pulling the submarine down. You can come to realize that the living quarters and these tanks that hold compressed air called "Ballast Tanks," when used in unison, can overcome this force of gravity.
Conclusion We have given you a general idea of how diving and surfacing works in both submarines and casual swimming. It was proposed that the force pushing an object back up, counteracting its fall is equal to the weight of the displaced fluid. Thus if you know w fluids density, the volume of that object submerged in that fluid (equal to the volume displaced), and the gravitational acceleration constant, you can calculate this force. Knowledge of these concepts leads to more sophisticated concepts, like calculating an objects terminal velocity in a certain fluid.
Marc
Archimedes Principle and Buoyancy
Some objects, when placed in water, float, while others sink, and still others neither float nor sink. This is a function of buoyancy. We call objects that float, positively buoyant. Objects that sink are called negatively buoyant. We refer to object that neither float nor sink as neutrally buoyant.
The idea of buoyancy was summed up by Archimedes, a Greek mathematician, in what is known as Archimedes Principle: Any object, wholly or partly immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
From this principle, we can see that whether an object floats or sinks, is based on not only its weight, but also the amount of water it displaces. That is why a very heavy ocean liner can float. It displaces a large amount of water.
Archimedes principle works for any fluid, but as divers we are mainly concerned with two different fluids: fresh water, and salt water. We need to think of fresh water and salt water as two different fluids because equal volumes of fresh water and salt water do not weigh the same. For example, a cubic foot of fresh water weighs approximately 62.4 lbs, while a cubic foot of salt water weighs approximately 64 lbs. The extra weight is because of the dissolved minerals in salt water.
Let's take a moment and look at an object in water and Archimedes Principle. If you place a 1 cubic foot object that weighs 63 lbs into fresh water, the object is displacing 62.4 lbs of water, but weighs 63 lbs. This object will be negatively buoyant - it will sink. It is however being buoyed up with a force of 62.4 lbs, so if we weighed it in the water it would only weigh .6 lbs.
If we put the same object into salt water, it would still weigh 63 lbs, but would be buoyed up by a force of 64 lbs, and it would float. It would be positively buoyant in salt water. To make the object neutrally buoyant in salt water, we would have to add 1 lb of weight to the object without changing its size (without changing is displacement). Then it would weigh 64 lbs, and be buoyed up with a force of 64 lbs, thus being neutrally buoyant.
Let's expand on this and look at an example of putting these ideas to work in a real situation. Suppose you know that a boat had lost an anchor weighing 100 lbs. Measuring a comparable anchor, we find out that the anchor displaces 1/2 cubic feet of water. We will also assume that the anchor was lost in a fresh water lake. You do a dive and find the anchor and want to bring it to the surface but the only resource you have available are some 1 gallon milk jugs. How many would you need to tie on to the anchor to float it to the surface?
At this point we need to do a little simple math. We know that a cubic foot of fresh water weighs 62.4 lbs, so the anchor displacing 1/2 a cubic foot of water would be buoyed up with a force of 31.2 lbs. Let's round this to 31 lbs for simplicity. This means our anchor that weighs 100 lbs on land will weigh 100-31 or 69 lbs in the water. We now know we need enough 1 gallon milk jugs to generate 69 lbs of lift.
Perhaps you remember the old expression "A pint a pound the world around." This refers to the fact that a pint of water weighs about a pound. Since there are 8 pints in a gallon, we know a gallon of water must weigh about 8 lbs. Since we know a cubic foot of water weighs 62.4 lbs, this means there are about 8 gallons of water in a cubic foot. Let's put it together and solve our anchor problem.
If we need 69 pounds of lift, we divide 69 by 8 lbs per gallon to learn we need 8.625 gallons of water displacement to make the anchor neutrally buoyant. This means, we could fill 9-one gallon milk jugs with air to lift our anchor.
Let's try another. A 3 cubic foot object weighing 400 lbs is dropped into the ocean. How big of an air lift bag (in cubic feet) would you need to lift the object?
First we determine that a 3 cubic foot object in salt water would have 3x64 lbs of lift, or 192 lbs of buoyant force. If we subtract 192 from 400 we get 208 lbs. This means we need to generate 208 lbs of lift to make our object neutrally buoyant. We then divide 208 (the objects in water weight) by 64 (the weight of a cubic foot of sea water) to get 3.25 cubic feet of displacement is needed to make the object neutrally buoyant. Thus, we would need at least a 3.25 cubic foot air lift bag.
Archimedes Principal
Imagine you jump into a swimming pool and start paddling with your feet. If you stop paddling, you begin to sink, and if you sink you dive to a certain depth. If you have air in your lungs, and if you relase this air as you submerge, you will dive faster. These ideas are explained in terms of physics, and the big principal is called "Archimedes Principal." This concept basically says that the net force pushing you back up, counteracting your fall/dive is equal to the weight of the fluid displaced, the fluid in this case being water. Of course you can apply the same idea to a falling man in the earth atmosphere, the fluid in that case is the air.
The idea is really simple to understand, but some explaining needs to be done. Imagine you are holding a rock at some height off the ground. If you release that rock, it falls at some speed. Well, Isaac Newton once said that the net force that pulls this rock down is equal to the objects (the rocks) mass times how fast it is accelerating:
F = M * A
But it might not me intuitively obvious that there is a force pushing the rock back up, counteracting its fall. This force can be thought of as the combination of the force of friction (since the rock collides with air molecules) and the force of buoyancy. It is easy to understand friction. Just slide on the carpet and try to figure out why you get burnt, or why your basketball stops rolling on a horizontal surface without given some initial push, and you not pushing it. Well, if you imaging all the molecules on the edge of some surface colliding with your object, then you realize what friction is. Just try running through a bush, you will come to realize that it is not easy because the bush resists your movement. Now the concept of buoyancy is not as intuitively obvious. Archimedes principal essentially explains this force of buoyancy. Basically, when you are falling through some fluid, the force that is pushing you back up, the force that keeps you from falling, in addition to friction, is this force of buoyancy. Archimedes principal says that this force is (once again) equal to the weight of the displaced fluid:
Fb = r * g * V
Fb = force of buoyancy
r = Density of the fluid being displaced
g = gravitational acceleration constant
V = volume of object submerged
As the diagram shows, if you assume that the objects center of gravity is in the center, then the force of gravity acts in such a way as to pull the object down and the force of buoyancy acts to pull the object up.
Diving and Surfacing, Submarines When an object is floating, the two forces acting on it (assuming force of friction is negligible, reasonable since the object is not moving), force of gravity (Fg) and force of buoyancy (Fb) are equal in magnitude, opposite in sign, hence they cancel about the objects center of mass. On the other hand, when the object gravitational force is greater than Fb, the object sinks. Lastly, when the objects buoyant force is greater than its Fg, it rises. This idea is applied to submarines. Submarines are heavy pieces of steel with torpedoes. They sink and surface by using the laws of physics and by using pumps and tanks that hold compressed air. When these tanks are full of compressed air, the submarine rises in elevation. When these tanks are empty and in some cases full of water, the submarine dives.
It is essential to understand that a submarine is not just a heavy piece of steel, for inside the submarine there is living area. Thus a good fraction of the submarine is composed of air, and only a small fraction of the total submarine is composed of heavy steel. Since steel is really dense and heavy, one needs a lot of air (hence, one needs to displace a lot of water) to counteract this force of gravity pulling the submarine down. You can come to realize that the living quarters and these tanks that hold compressed air called "Ballast Tanks," when used in unison, can overcome this force of gravity.
Conclusion We have given you a general idea of how diving and surfacing works in both submarines and casual swimming. It was proposed that the force pushing an object back up, counteracting its fall is equal to the weight of the displaced fluid. Thus if you know w fluids density, the volume of that object submerged in that fluid (equal to the volume displaced), and the gravitational acceleration constant, you can calculate this force. Knowledge of these concepts leads to more sophisticated concepts, like calculating an objects terminal velocity in a certain fluid.
That would be SimKnife, a new computer game put out by you know, whoever put out the other useless Sim games.
Heh, dont take my name in vain - i wouldnt call Andy CantH2AddyUp now would i? This of course goes along with his other alter-egos of NotH2Andy and H2Andrea depending on the situation!NetDoc:That would be SimKnife, a new computer game put out by you know, whoever put out the other useless Sim games.
Pete i wont make fun of your name as i will be relying on you in the team next weekend down in another dark, although pleasantly sunlit, we hope, hole. BTW, i never really got into that whole spending your life living vicariously through watching stuff happen on a computer screen kind of game - real life is far more interesting. However if i am SimBrooks, who is pulling my strings?
FLL Diver:
EUREKA!
now get out of the bath tub
glbirch:
Then if I keep it on uniil I get some ice. Up I go. Even colder yet.
I feel sorry for those folks that don't understand the pleasures of 4C water.
Now to combine my diving fridge with a rebreather...... :06:
scbababe
Guest
FLL Diver:
:11: Oh No.. It burns, turn it off! It burns! I could not have quoted it better my self! :dazzler1:
widebody50
Contributor
simbrooks:Ummm, read my other thread on red tape and immigration, the doors are most definately jammed shut, locked and guarded to decent law abiding citizens! Harder to get into this country legally than Fort Knox!
Well, you ARE HERE, aren't you??? I am all too familiar with "La Migra". as it is known here in SoCal. My wife is from Mexico, and we had a devil of a time getting her permanent resident status. Fortunately, all is well now.
On the other hand, coming here legally, and expecting to stay and work, well, it does become much more difficult, just as in most other countries. Just do what many people do...come in on a visitor visa, and work under the table until the government offers another amnesty program. With GW in the White House, anything could happen.
RW
widebody50
Contributor
H2Andy:
Thanks H2...now I can buy that new BC I've been drooling over!!!
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