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skprbob · 10-12-2006, 11:35 AM

The two-word answer to Dave R and Yankee: Archimedes' Principle.

The more detailed answer:

A 4 ft x4 ft x4 ft block of anything, fully submerged, displaces 64 cubic feet of water. Fresh water has a density of 62 lbs/cubic ft. By Archimedes' Principle, the block would thus have a buoyant force on it of 64 x 62, or 3970 lbs. If the block itself weighed less than that, the buoyant force would be greater, pushing the bnlock up, and the block would float. In fact it would rise out of the water until its weight and the weight of the water it displaces were equal, then sit there floating peacefully; so-called positive buoyancy. If the block's weight and the buoyant force were equal, the block would submerge completely and remain at whatever depth one wished to place it at; so-called neutral buoyancy and very hard to maintain for a number of reasons - just ask a SCUBA diver! If the block's weight was greater than the buoyant force, the block would submerge completely and continue to sink, since the downward force is greater; so-called negative buoyancy.

Example - Concrete has a density of roughly 100 lbs/cubic foot, so a 4x4x4 block would weigh 6400 lbs. That's greater than the buoyant force on it if it were completely submerged, so it sinks. It would also weighs only 6440 - 3970, or 2430 lbs underwater. You have to pour a block weighing over 3 tons in order to get just over one ton of holding weight!

10-12-2006, 11:35 AM	#19
skprbob Senior Member Join Date: Sep 2006 Location: 19 Mile Bay Posts: 147 Thanks: 0 Thanked 90 Times in 29 Posts	granite The two-word answer to Dave R and Yankee: Archimedes' Principle. The more detailed answer: A 4 ft x4 ft x4 ft block of anything, fully submerged, displaces 64 cubic feet of water. Fresh water has a density of 62 lbs/cubic ft. By Archimedes' Principle, the block would thus have a buoyant force on it of 64 x 62, or 3970 lbs. If the block itself weighed less than that, the buoyant force would be greater, pushing the bnlock up, and the block would float. In fact it would rise out of the water until its weight and the weight of the water it displaces were equal, then sit there floating peacefully; so-called positive buoyancy. If the block's weight and the buoyant force were equal, the block would submerge completely and remain at whatever depth one wished to place it at; so-called neutral buoyancy and very hard to maintain for a number of reasons - just ask a SCUBA diver! If the block's weight was greater than the buoyant force, the block would submerge completely and continue to sink, since the downward force is greater; so-called negative buoyancy. Example - Concrete has a density of roughly 100 lbs/cubic foot, so a 4x4x4 block would weigh 6400 lbs. That's greater than the buoyant force on it if it were completely submerged, so it sinks. It would also weighs only 6440 - 3970, or 2430 lbs underwater. You have to pour a block weighing over 3 tons in order to get just over one ton of holding weight!