Displacement of Scooters at Depth - Spun off from the A&I Discussion about Nothernone

[Steve_C]

Well, do the following test. Get 2 Coca-Cola cans. Open one of them, drink it & crush it down to a wafer. May be even hammer it down as much as you can to get rid of any air space in it. Then throw both the crushed Coca-Cola cab together with the unopened one. Which one would sink faster?

If both cans have no air in them, I bet you they will sink about the same time.

Well it could also depend on whether this is done in fresh or salt water. Assuming that coke has the density of fresh water, then the full can in salt water would have some buoyancy.

 

[pepperbelly]

When there is no air trapped in the scooter, regardless whether it is fully flooded scooter or imploded, should be about the same.

Archimedes principle includes the weight of the container, not just the weight of the water being displaced. Why do you think a full oil tanker float higher than the empty one?

It does?

I wasn't told there would be math.

 

[ChuckP]

This pains me to see this - it's simple physics.

Scooter weighs 20kgs on land at sea level
Intact scooter under seawater displaces approx 20 liters of seawater - it's neutrally buoyant.

Flooded, imploded or damaged scooter under seawater displaces 6 liters of seawater - what's the scooters relative weight under seawater now? 20kgs (original weight) minus 6kgs (6 liters of displaced seawater) equals 14kgs.

The above calculation doesn't take into account any material used in making the scooter that has a specific gravity less than one (actually less than 1.02) - that material floats anyways, so it'd effect actual weight under water and make it lighter - heavy batteries have no bearing on the weight under water.

Weights are purely made up and don't relate to any scooter - the one I picked up the other day was heavy......

 

[Dan]

Well it could also depend on whether this is done in fresh or salt water. Assuming that coke has the density of fresh water, then the full can in salt water would have some buoyancy.

May be tiny whinny bit more, too small to make a difference. According to Seawater - Wikipedia surface seawater density varies between 1.020-1.029 g/ml depending upon the salinity and temperature. That's about 2-3% heavier than freshwater. The density of aluminum is more than twice of the water (about 2.7 g/ml), so both would sink like a rock. Then the crushed-can wafer would have more drag if it falls horizontally. That might cancel the less buoyancy. I wouldn't be able to tell the difference in speed.

 

[CT-Rich]

Reading over these several pages, I am somewhat mortified at at some of the misconceptions about why thing float or sink, by people who I would think have a vested interest in understanding it.

 

[Dan]

I don't know the air volume of the scooter he used, but I can make a comparison to a scuba tank, which has an internal volume that is 100% air. I am looking at a chart as I write and rounding everything off for ease of understanding.

The scuba tank weighs roughly 35 pounds on shore when empty.
When empty, it is neutrally buoyant, meaning it has no apparent weight in water.
It's physical capacity is 11 liters.
A liter weighs about the same as a kilogram of water.
11 kilograms of water = 24 pounds.

If filled with water, this scuba tank would weigh about 24 pounds in water as opposed to 35 pounds on land.

I think it'll weigh 11 (35 - 24) pounds in water.

 

[Dan]

...Edit: I see what you did there. If empty it was neutral, then flooded it would be negative by the weight of the water. For that case you are correct and if you know the volume and the negative or positive buoyancy empty you could easily calculate what a flooded cylinder would weigh under water. Sorry.

If the tank empty weight is 35 lbs and filled up with 24 lbs (11 liters) of seawater, then the tank would weigh 11 (35 -24) lbs underwater.

 

[Compressor]

Review of Archimedes principle: Any object, totally or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

Fb = ρ x g x V Where Fb= buoyant force; ρ is density g is gravity and V is volume.

Hope this helps.

 

[johndiver999]

If the tank empty weight is 35 lbs and filled up with 24 lbs (11 liters) of seawater, then the tank would weigh 11 (35 -24) lbs underwater.

If the tank is neutral in the water and then you flood it, it will then have a weight equal to the weight of the water which has been added to the inside. It is unrelated to the weight of the vessel itself...

 

[Dan]

If the tank is neutral in the water and then you flood it, it will then have a weight equal to the weight of the water which has been added to the inside. It is unrelated to the weight of the vessel itself...

People tend to forget about the gravity force (i.e., the tank weight) that goes against the buoyancy force. We are still on earth. Once you are in the water, it doesn't mean that the gravity force would disappear. Buoyancy force is up & gravity force is down. If the gravity force (35 pounds) is greater than the Buoyancy force (24 pounds) then you will have negative (down / sink) buoyancy force of 11 pounds.

Buoyancy - Wikipedia

Archimedes' principle:
Apparent immersed weight = object weight - weight of displaced fluid

If you put that tank in a pool of liquid is Mercury with density of 13.6 g/ml (13.6 times greater than water) the buoyancy force would be 13.6 x 24 pounds = 326.4 pounds >> tank weight of 35 pounds. That tank will float as demonstrated by the picture, below (placing a nickel on a beaker of mercury). The coin density perhaps is about 8.9 g/ml < Mercury density of 13.6 g/ml.

Another way of estimating whether an object is positively or negatively or neutrally buoyant is by estimating its bulk density relative to the fluid density around it. If the bulk density is 1.00 g/ml (62.4 lbs/cft) then it'll be neutrally buoyant in freshwater. For the case of empty AL80, if its internal volume = 11 liters + 5 liters of aluminum material = 16 liters of outside volume = 0.57 cft. Then the empty AL80 bulk density = 35 lbs / 0.57 cft = 61.4 lbs/cft, pretty close to water density to me. It'll be slightly floating if the nozzle capped, I think.