Ewen Posted March 4, 2010 Share Posted March 4, 2010 It was one of mine. Closed now. Theres a barge floating in a giant tank of water. The barge is carrying some large blocks of concrete. The barge weighs 300 tonnes, the concrete blocks together weigh 100 tonnes. The barge cranes the concrete blocks out of the hold, and lowers them into the water alongside it. Does the water level in the tank now rise, fall or stay the same ? Quote Link to comment Share on other sites More sharing options...
Thorin Posted March 4, 2010 Share Posted March 4, 2010 It was one of mine. Closed now. Theres a barge floating in a giant tank of water. The barge is carrying some large blocks of concrete. The barge cranes the concrete blocks out of the hold, and lowers them into the water alongside it. Does the water level in the tank now rise, fall or stay the same ? The barge will take off? Quote Link to comment Share on other sites More sharing options...
SupraStar 3000 Posted March 4, 2010 Share Posted March 4, 2010 depends on the surface area of the tank of water. Quote Link to comment Share on other sites More sharing options...
SupraStar 3000 Posted March 4, 2010 Share Posted March 4, 2010 is it a mexican riding a bike carrying a french loaf? Quote Link to comment Share on other sites More sharing options...
tbourner Posted March 4, 2010 Share Posted March 4, 2010 depends on the surface area of the tank of water. Depends on the density of the concrete compared to the water I would have thought? Quote Link to comment Share on other sites More sharing options...
Ewen Posted March 4, 2010 Author Share Posted March 4, 2010 depends on the surface area of the tank of water. Can be any size of tank, any size of barge, any number of concrete blocks. Quote Link to comment Share on other sites More sharing options...
Ewen Posted March 4, 2010 Author Share Posted March 4, 2010 Depends on the density of the concrete compared to the water I would have thought? Density is the key to the answer. I've edited the wording of the puzzle to give a further clue. Quote Link to comment Share on other sites More sharing options...
Pixelfill Posted March 4, 2010 Share Posted March 4, 2010 Density is the key to the answer. I've edited the wording of the puzzle to give a further clue. please try again Ewen, it got lost during my tidy up. Mike Quote Link to comment Share on other sites More sharing options...
spidermonkey Posted March 4, 2010 Share Posted March 4, 2010 The water will fall Quote Link to comment Share on other sites More sharing options...
SupraStar 3000 Posted March 4, 2010 Share Posted March 4, 2010 Depends on the density of the concrete compared to the water I would have thought? I think we can assume the concrete is dry and was made of a ratio of 1:3:6 Quote Link to comment Share on other sites More sharing options...
Rob Posted March 4, 2010 Share Posted March 4, 2010 IIRC, the water level falls because the displacement of water from the laden hull is greater than the displacement from the concrete blocks themselves in the water. Or summat or nuffink. Quote Link to comment Share on other sites More sharing options...
JamieP Posted March 4, 2010 Share Posted March 4, 2010 Water will fall. Quote Link to comment Share on other sites More sharing options...
Wez Posted March 4, 2010 Share Posted March 4, 2010 I am rubbish at stuff like this but here goes anyway Stay the same? The concrete blocks in the barge would force the barge to displace more water due to the extra weight and then when they are moved to the water the blocks would displace the water but the barge would displace less, the density of the blocks would be the same regardless if they are in the barge or in the water. Erm, maybe it falls Quote Link to comment Share on other sites More sharing options...
tbourner Posted March 4, 2010 Share Posted March 4, 2010 The blocks are more dense than the water (so they sink), so the weight of the blocks (when in the barge) has more displacement effect than the volume of the blocks (when in the tank). Water level falls. Quote Link to comment Share on other sites More sharing options...
Ewen Posted March 4, 2010 Author Share Posted March 4, 2010 I think you all have it now... Its all about density. In this case density of the barge and concrete relative to the density of water. The barge weighs 300 tonnes say, and its carrying 100 tonnes say, of concrete blocks. The barge floats.... so the density of each cubic metre of barge hull volume thats under the water must be the same as the density of each cubic metre of the water it floats in. Assuming the density of the water is 1 tonne per cubic metre, the barge and its load of concrete is displacing 400 cubic metres of water. The concrete blocks dont float, as we know their density is far higher than 1 tonne per cubic metre. Once the concrete blocks are off the barge and into the water, the cubic volume of 100 tonnes of concrete is now displacing far less than the cubic volume of 100 tonnes of water. So, overall, once the concrete is not being supported by a floating barge, less volume of water is being displaced and the water level in the tank will fall. Quote Link to comment Share on other sites More sharing options...
Ewen Posted March 4, 2010 Author Share Posted March 4, 2010 Four barges this time... Four giant tanks of water, each with a barge floating in them. Tank 1 has a barge made of wood in it. Tank 2 has a barge made of aluminium in it. Tank 3 has a barge made of steel in it. Tank 4 has a barge made of lead in it. Each tank is exactly the same size. Each tank contains the same amount of water. Each barge weighs say, 400 tonnes. Which tank has the highest water level ? Quote Link to comment Share on other sites More sharing options...
supra_aero Posted March 4, 2010 Share Posted March 4, 2010 If they all weigh the same, surely the water level will be equal assuming nothing is being removed Quote Link to comment Share on other sites More sharing options...
tbourner Posted March 4, 2010 Share Posted March 4, 2010 How can they all be the same size, and weigh the same, and be made of different materials? Quote Link to comment Share on other sites More sharing options...
Wez Posted March 4, 2010 Share Posted March 4, 2010 How can they all be the same size, and weigh the same, and be made of different materials? Maybe they have different thickness walls Quote Link to comment Share on other sites More sharing options...
Ewen Posted March 4, 2010 Author Share Posted March 4, 2010 How can they all be the same size, and weigh the same, and be made of different materials? I didnt say the barges were the same size. Quote Link to comment Share on other sites More sharing options...
tbourner Posted March 4, 2010 Share Posted March 4, 2010 Maybe they have different thickness walls Oh OK, the same external dimensions, not exactly the same size (as quoted in the question). //edit: bum. MLTR. Quote Link to comment Share on other sites More sharing options...
Ewen Posted March 4, 2010 Author Share Posted March 4, 2010 I've always loved this stuff since a kid. Why wood floats yet ships made of wood can sink, why steel sinks yet boats made of steel can float and so on. The principles are all the same, including the one that explains why the concrete block dangling off the crane above the water weighs less when its under the water. Quote Link to comment Share on other sites More sharing options...
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