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  #11  
Old 04-20-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

Former Fiziks instructor Frogwatch here:

Density of lead is roughly 12 gm/cm3
Density of concrete is roughly 2.5 gm/cm3

100 lb ~45 Kg =45000 gm For lead, this means that it occupies 3750 cm3 (divide the 45000 gm by 12 gm/cm3).
For concrete,100 lb occupies 18000 cm3. So, you can see that the same weight of concrete occupies 4.8 times the volume of lead (the lead would occupy a cube 15 cm on a side while the concrete would occupy a cube of 26 cm on a side)
Now, just like you used to do in basic physics, draw an up arrow and a down arrow. The down arrow should be labelled mg and the up arrow should be labelled Fb (buoyant force).
Buoyant force is equal to the weight of water displaced so for the case of lead, the total force is then 45000g-3750g =41250g. For concrete, it is 45000g-18000g = 27000g.
Thus, the 100 lb of lead "weighs" more than the 100 lb of concrete in water. However in both cases, the mass is the same 45 kg. The confusion arises from the definition of weight vs mass and that we commonly interchange weight and mass however THEY HAVE DIFFERENT UNITS. Weight is a unit of force that varies depending on the place it is measured whereas mass does not change from place to place (Newtonian physics here).
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by Capt Len View Post
It's called specific gravity and all about weight for a given VOLUME , sort of a eureka moment. Since a cubic foot of ocean weighs 64 lbs (2 lbs of salt) a cubic foot of concrete weighs 64 lbs less in the water, If it's in the bilge it's all the same but your metacentric heights change but that's another story.
Specific gravity is the density of something relative to the density of a reference material (usually pure water). In this case it's a bit more convenient to reference density directly, since the frame of reference keeps changing.
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Re: 100 lb of Lead Vs 100 lb of concrete

OK, I mis-remembered the density of lead as being 12 when it is really 11.3. However, the density of mercury is 13. or so. I once picked up a pint bottle of mercury and almost immediately dropped it. That thing was seriously heavy. I once had a brick sized block of depleted U as a door stop painted to look like a normal brick. Seriously heavy even compared to those of us familiar with lead bricks. Gold is seriously dense. Movies where the guy casually picks up a gold brick.............no way. Pure gold is so soft that it makes lead look like steel, you can scratch it just by thinking about it.
The ultimate ballast, Osmium. I just bought 10 gms of the stuff for optical coatings. Densest element, very hard, will not corrode, rarer but cheaper than gold
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Re: 100 lb of Lead Vs 100 lb of concrete

There is a metal called Gallium that melts at room temp and it looks a lot like Mercury but it is non-toxic. Unfortunately, its density is only 5.9 (iron is roughly 8.9). However, water is only a density of 1. So...................
A combo water ballast/pumpable gallium ballast. A hollow reservoir is mounted on the to-rails on an extension to get it about a foot away from the boat. The one on the downhill side serves to provide buoyancy to that side whereas the other one is filled with Gallium that has been pumped there. Most ballast would be water ballast but the use of Gallium only serves to minimize the size of these reservoirs.
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by Maine Sail View Post
Yes, it is correct. It's all about the density of the material. Some areas even specifically ban the use of concrete for moorings because its underwater weight is a /.55 factor....
Some areas? Maine, for instance?....where the only thing to use underwater is a big block of Maine granite? I'm surprised you guys aren't exporting the stuff.
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by Frogwatch View Post
...
The ultimate ballast, Osmium. I just bought 10 gms of the stuff for optical coatings. Densest element, very hard, will not corrode, rarer but cheaper than gold
It's also extremely toxic, at least its oxides and salts are. (I used to have to deal with osmium compounds for electron microscopy work.)
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Re: 100 lb of Lead Vs 100 lb of concrete

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A combo water ballast/pumpable gallium ballast. A hollow reservoir is mounted on the to-rails on an extension to get it about a foot away from the boat. The one on the downhill side serves to provide buoyancy to that side whereas the other one is filled with Gallium that has been pumped there.
Or just get a fat kid to sit on the rail. Less to go wrong, easier to find another one if you break it. Less pollution if it leaks too.
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Re: 100 lb of Lead Vs 100 lb of concrete

The weight of a body in water is equal to the items weight minus the water is displaces. So, if 100 lbs of lead is smaller and displaces 10 lbs of water, its weight is 90 lbs. If a 100 lb block of concrete which is larger, displaces 20 lbs of water, its weight is 80 lbs. In the same way, a boat "displaces" 6700 lbs, it will drop into the water until the amount of water that it pushes out of the way is 6700 lbs.
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by PaulinVictoria View Post
Or just get a fat kid to sit on the rail. Less to go wrong, easier to find another one if you break it. Less pollution if it leaks too.
Jeesh, now you tell me .... I'll just chop of that darn keel, eat more and sit on the rail. That way the pesky rocks close to shore can't jump out and play tag with me.
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Re: 100 lb of Lead Vs 100 lb of concrete

I think the fat kid needs more maintance... however, depending on the age of the kid, he might be as dense as lead
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