Quiz - Physics - Minimum Displacement

[elmo]

I'm just wondering how they had such lightweight concrete. Pumice in the aggregate mix?

 

[Joneill]

Spoiler: Answer

d. 300 l/10.63 cf

I got that answer from the choices given - but, technically, 300 l would make it neutral - so wouldn’t you actually need a tad more than 300 l to “move the block off the bottom”? Otherwise, won’t it just sit where it’s at?

 

[Couv]

I got that answer from the choices given - but, technically, 300 l would make it neutral - so wouldn’t you actually need a tad more than 300 l to “move the block off the bottom”? Otherwise, won’t it just sit where it’s at?

Plus the weight of the lifting device and the tackle.

 

[elmo]

I got that answer from the choices given - but, technically, 300 l would make it neutral - so wouldn’t you actually need a tad more than 300 l to “move the block off the bottom”? Otherwise, won’t it just sit where it’s at?

Take a breath or add add a squirt to your BC to become a tiny bit positive then breathe out or dump gas to put it back down again. If you inflate the lift bag to be positive you risk a runaway mooring block.

 

[Ukmc]

You would need to overcome suction created by the block in the substrate, which really falls outside the parameters of the question and really has nothing to do with buoyancy of the block. Depth would be relevant only from the perspective of how many tanks would be required to fill the lift bag to launch it towards the surface, which might be a reasonable follow up question.

Ha, have you ever thought of how much force is required to overcome what you describe as a suction effect. That is was exactly my point. If you have a block with completely flat surface laying on a flat bottom, there is a significant force required to overcome this “suction effect” which will be directly proportional to the depth. Once overcome, 600 liters of water displacement will be required to float 600 kg object.

 

[Ukmc]

I don’t think that’s correct or you’d be in trouble if you touched the bottom

Haha. That is because you are not shaped as a concrete block. However, if you lay flat in a silt (assuming there is no gear to interfere with your surface to surface interaction) one may have difficulty lifting you up if you maintain you flat surface like a concrete block
I did get the answer right, but there are real life considerations as others mentioned that need to be taking into account

 

[ToneNQ]

I got that answer from the choices given - but, technically, 300 l would make it neutral - so wouldn’t you actually need a tad more than 300 l to “move the block off the bottom”? Otherwise, won’t it just sit where it’s at?

There are probably 50 additional things to consider, if we want to get technical. It's standard procedure for cranes to calculate the weight of their rigging into the load, same applies here. We can calculate the theoretical buoyancy, then we'll learn the actual buoyancy of this 300L of concrete.

 

[CT-Rich]

Ha, have you ever thought of how much force is required to overcome what you describe as a suction effect. That is was exactly my point. If you have a block with completely flat surface laying on a flat bottom, there is a significant force required to overcome this “suction effect” which will be directly proportional to the depth. Once overcome, 600 liters of water displacement will be required to float 600 kg object.

Depth has nothing to do with suction and very little to do with buoyancy. If it is sitting on rock or porous sand or very fine silt or clay. The pressure is always going to be relative to the surrounding water. A diver at 90’ can’t really tell he is at 90’ by pressure. That is why you bring a gauge. If you bring a closed milk jug to 90’ it will be affected by pressure in a way the diver (or concrete block) won’t.

 

[emoreira]

This one's simple if you stay metric. Imperial makes you do some arithmetic.

I love metric system. Math is so simple.

 

[Joneill]

Take a breath or add add a squirt to your BC to become a tiny bit positive then breathe out or dump gas to put it back down again. If you inflate the lift bag to be positive you risk a runaway mooring block.

I get that - but the question was about the lift bag moving the block off the bottom - exactly 300 l won’t quite get it to move off of the bottom