Quiz - Physics - Minimum Displacement

[Couv]

Well yes, you don't fully inflate your lift bag, just enough to get it to inflate. Then you dump gas as you ascend to counter the lower pressure/expansion of gas.

Or even more practical: Use an open bottom lift bag rated to lift just above the weight of the object you want to lift. As the gas expands in the bag it spills out of the bottom and maintains the same amount of lift on it's own.

 

[wetb4igetinthewater]

Or even more practical: Use an open bottom lift bag that is rated to lift just above the weight of the object you want to lift. As the gas expands in the bag it spills out of the bottom and maintains the same amount of lift on it's own.

that's ideal if you have that. I have a couple 50 lb lift bags, 25, 70, 100, 200 ones (yes, I've moved a fair bit of stuff underwater). Don't ask me how many DSMBs I have. Some are open bottom, others are not.

 

[Storker]

Use an open bottom lift bag that is rated to lift just above the weight negative buoyancy of the object you want to lift.

FTFY.

And personally, I see some challenges to that approach. Primarily how to calculate the proper rating of said lift bag accurately enough. Secondarily, how and where to source a lift bag with just the correct rating for just that job.

 

[Ukmc]

It seems like a question is testing archimedes principle. 300 L block displaces 300 L volume of water which weighs 300 kg and you need another 300L/300kg bag to make it float. In that case the depth shouldn’t matter. I maybe wrong, but when a block is laying on the bottom, such as silt, Archimedes principle should not apply as the only force that is applied and needs to be overcome is the column of water and air (until it has been lifted enough off the bottom for Archimedes to kick in). In that scenario, the depth is important (and supplied) but the area of the surface of the block is not given ...

 

[Pedro Burrito]

Spoiler: Answer

d. 300 l/10.63 cf

 

[rhwestfall]

yep!

1350 - (11 x 62.4) = 663 that is needed to overcome to make neutral
663 / 62.4 = 10.6346

Fun!

 

[CT-Rich]

It seems like a question is testing archimedes principle. 300 L block displaces 300 L volume of water which weighs 300 kg and you need another 300L/300kg bag to make it float. In that case the depth shouldn’t matter. I maybe wrong, but when a block is laying on the bottom, such as silt, Archimedes principle should not apply as the only force that is applied and needs to be overcome is the column of water and air (until it has been lifted enough off the bottom for Archimedes to kick in). In that scenario, the depth is important (and supplied) but the area of the surface of the block is not given ...

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.

 

[Angelo Farina]

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.

Which was yesterday's question...
Quiz - Physics - Fill a Container at 40 metres/132 feet

 

[BlueTrin]

It seems like a question is testing archimedes principle. 300 L block displaces 300 L volume of water which weighs 300 kg and you need another 300L/300kg bag to make it float. In that case the depth shouldn’t matter. I maybe wrong, but when a block is laying on the bottom, such as silt, Archimedes principle should not apply as the only force that is applied and needs to be overcome is the column of water and air (until it has been lifted enough off the bottom for Archimedes to kick in). In that scenario, the depth is important (and supplied) but the area of the surface of the block is not given ...

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

 

[Scraps]

UKMC:

It's not that Archimedes' principle kicks in or fails to kicks in (according to my Google machine, the buoyancy force is always present whether an object floats, sinks or remains suspended). It's that other forces can also be at work in the real world.

For example, a ship with a 28-foot draft is pushed up by a force equal to the mass of the water it displaces. However, if that same ship accelerates as it steams along a narrow 30-foot deep channel, it will start to ride lower, or squat, in the water. Why? Is Archimedes fading out? No. Another force, in this case the Venturi effect, has come into play.

So if silt or muck or suction or some other factor affects your hypothetical concrete block, it just means you successfully resisted the test question's attempt to isolate the one factor it was asking about.