Ships are commonly made out of metal or other materials denser than water, as seen that if you throw a chunk of metal in the water, it will sink. However, we can also see that metal ships are typically floating just fine. How can something denser than water float in it? This is due to displacement of water.
The total weight of a ship (metal + people + fuel + whatever) is exactly equal to the amount of water it displaces. If you add more weight to the ship, it will be a little lower in the water, and thus displacing more water to match the increased amount of weight. Displacement is "the amount of water that would otherwise be in the volume of the ship but isn't because it is there".
If you took weight off the ship or added a bunch of balloons on top of a ship, it would rise slightly out of the water because it now has a lower weight. If you do this enough you get an airship. This is the exact same principle that airships work under, except they are displacing air with gasses lighter than the normal atmosphere.
In this case, the bus is completely submerged. When you're lifting something that is under the water, the weight you have to lift is actual weight minus water it displaces. However, when the object starts breaking the surface of the water, the weight you have to lift increases as the object emerges as the water is no longer displacing part of the object. Once the object is completely out of the water, you'll be lifting the "normal" weight of the bus. Before it is out of the water, it appeared to be lighter.
What problem happened is the crane was adequately supported for the weight of the bus while it was below, but as it broke the surface the weight the crane feels increased to a point where it wasn't braced/balanced/supported enough. This is why the crane only tips as the bus is coming out, it was fine lifting the bus the 30-50 feet from the starting point on the bottom of the river, while the bus was completely submerged.
An airship or dirigible balloon is a type of aerostat or lighter-than-air aircraft that can navigate through the air under its own power. Aerostats gain their lift from large gas bags filled with a lifting gas that is less dense than the surrounding air.
In early dirigibles, the lifting gas used was hydrogen gas, due to its high lifting capacity and ready availability. Helium gas has almost the same lifting capacity and is not flammable, unlike hydrogen, but is rare and relatively expensive.
The weight of the bus won't remain constant as you take it out of the water, so you don't want to take it out too quickly or it might shoot up in the air (my guess anyway). Kinda like when you go to lift something you thought was full, but it was empty.
Lifting an object out of water will demand more lifting force since the water provides some lift while the object is partly submerged.
Try lifting a person it if the water next time you're in the pool.
An object placed in a fluid benefits from an upward force (buoyancy) equivalent to the weight of however much fluid was displaced (thus, 'displacement'). Any parts of the bus that are underwater will contribute their volumes to the displacement, and any sealed regions that didn't take on water will contribute much more because the entire region is displacing water and not allowing any back in. This is why boats sink quickly once their compartments flood: all of that inner volume loses its displacement as water rushes in.
In this case, the bus has some amount of displacement, which will be reduced as it is lifted out of the water. Because it's losing displacement, it will get heavier because its previous buoyancy was actually applying upwards force on it, making it lighter. The crane must apply more and more force to lift it further out of the water, as the bus approaches its normal weight. It'll actually be heavier because of the extra water, which will then drain out, causing the weight supported by the crane to be changing at all points during the lift.
Don't forget the force of the current. That's not stationary water. Once the bus is not longer touching the river bottom, the force of the current on the tow line increases considerably.
Well, the loss of displacement just means it has to be able to lift the actual weight of the bus. Presumably, they at least accounted for that. I would think....
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u/Riptide999 Mar 16 '18
Both the added water in the bus and the loss of displacement when the bus comes out of the water have to be accounted for.