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  #21  
Old 04-21-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by mad_machine View Post
I think the fat kid needs more maintance...
True. The lead is pretty much a "set it and forget it" sort of thing; but you'll probably have to keep feeding the fat kid sandwiches and beer all day.

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Originally Posted by mad_machine View Post
however, depending on the age of the kid, he might be as dense as lead
Even truer.
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  #22  
Old 04-21-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by SlowButSteady View Post
That's odd. I thought this thread was about concrete and lead.
Slow...
With the signature you are using, all of your posts belong in the "off topic" thread category. Your "Che Guevarra" type Obama signature also fell into the same category.
I suggest we all try not to incite anger in the rest of the forum members.
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  #23  
Old 04-21-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

That settles that, thanks for the input everyone.


And Slow..
I agree with seaduction, your signature line is not the place for your political objectives.
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  #24  
Old 04-21-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

Volume and displacement aside, in air 100# of lead and 100# of concrete both weigh the same 100#. The chunk of lead is much smaller. immersed in the water it ain't so.

["Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent."]

http://www.cement.org/basics/concret...retebasics.asp

Since concrete may be as much as 20% water, that "weight" comes off the top when measuring it with a scale when immersed in water. The guys at the marina were right.

Down

Last edited by downeast450; 04-21-2013 at 12:19 PM.
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Old 04-21-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

Hang anything that sinks on a large spring scale suspended above water... as you lower it into the water the object will 'lose' weight on the scale. Regardless of what the object weighs, it will 'lose' as much weight as the equivalent volume of the water - ie as much as it displaces.

A 700 lb cu ft of lead would 'lose' 62.5 lbs approx.... as would a cu ft of concrete or anything else.
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by downeast450 View Post
Volume and displacement aside, in air 100# of lead and 100# of concrete both weigh the same 100#. The chunk of lead is much smaller. immersed in the water it ain't so.

["Typically, a mix is about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water. Entrained air in many concrete mixes may also take up another 5 to 8 percent."]

Concrete Basics | Portland Cement Association (PCA)

Since concrete may be as much as 20% water, that "weight" comes off the top when measuring it with a scale when immersed in water. The guys at the marina were right.

Down
Actually, once the concrete cures, the water that was added into the original mix is mostly incorporated into the crystalline structure of the newly formed "rock". When Portland cement is manufactured the water is driven off, leaving an anhydrous dust (mostly calcium and silicon oxides). When water is added to this dust it re-hydrolyzes these oxides forming pretty complex crystalline lattice that varies a bit depending on the exact amount of water in the cure. The point is that the water is not "free water". It is chemically bonded to the rest of the concrete. So you can't really talk about it's density, as it has no real density per se.
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Old 04-22-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

Okay, I'm cancelling my concrete order.
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Old 04-22-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

Here's one for the engineers.

Does an object really weigh less once submerged? That implies an elimination of mass. It is subject to an opposing force (buoyancy). Does an aircraft weigh less when airborne because of lift?

Having taught deep wreck diving when I was younger, I get the concept being discussed. Just a question of technical clarity on whether something actually weighs less. I don't think so. I think it's really a case of opposing forces that makes the object appear to weigh less.
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Old 04-22-2013
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Re: 100 lb of Lead Vs 100 lb of concrete

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Originally Posted by Minnewaska View Post
Here's one for the engineers.

Does an object really weigh less once submerged? That implies an elimination of mass. It is subject to an opposing force (buoyancy). Does an aircraft weigh less when airborne because of lift?

Having taught deep wreck diving when I was younger, I get the concept being discussed. Just a question of technical clarity on whether something actually weighs less. I don't think so. I think it's really a case of opposing forces that makes the object appear to weigh less.
It all depends on what definition one is using: mass, weight, or apparent weight. Mass is a measure of the inertia of an object. Weight is the force an object exerts in a gravitational field (weight = mass x gravity; we talk about an object "weighing" so many kilograms, but in fact the proper units of weight are newtons). For a body at rest, apparent weight takes buoyancy into account. The buoyancy provided by air is small enough, and gravity is constant enough, that our operational use of apparent weight to estimate mass is sufficient for most applications. However, as this thread demonstrates, there are exceptions.
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Last edited by SlowButSteady; 04-22-2013 at 06:48 AM.
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Re: 100 lb of Lead Vs 100 lb of concrete

Cool. If weight = mass x gravity, then an object does not weigh less when buoyed, it just appears to.
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