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

[dmaziuk]

When you flood the cylinder it displaces less water.

Why?

 

[BRT]

Why?

Because the inside is not displacing water.

 

[dmaziuk]

Because the inside is not displacing water.

I see. So a cylinder filled with water displaces less water than a cylinder filled with mercury?

 

[Steelyeyes]

I see. So a cylinder filled with water displaces less water than a cylinder filled with mercury?

It's about the density of what's in the tank in comparison to the surrounding water. Sea water in the tank is the same density as the water outside the tank. Only the tank material has a different density that impacts buoyancy.

 

[Joneill]

I see. So a cylinder filled with water displaces less water than a cylinder filled with mercury?

I believe he meant that the inside of a flooded cylinder does not displace any water once it’s flooded and open to the surrounding water....

 

[dmaziuk]

It's about the density of what's in the tank in comparison to the surrounding water. Sea water in the tank is the same density as the water outside the tank. Only the tank material has a different density that impacts buoyancy.

I wasn't asking about buoyancy, I was asking about displacement. Displacement is volume and an object filled with air displaces exactly as much as the same object filled with water. I honestly don't know what happens (EDIT) to displacement when the object is open to surrounding water, I'm pretty sure its volume doesn't change.

I do know what happens to buoyancy so I am not asking that.

 

[Dominik_E]

Any volume only generates buoyancy when it is filled with something lighter than the surrounding medium that is displaced. When the cylinder is flooded, the light stuff (air) is lost and replaced by the same type of matter that is displaced (water). The buoyancy of the volume that is flooded thus goes to zero.

 

[boulderjohn]

Any volume only generates buoyancy when it is filled with something lighter than the surrounding medium that is displaced. When the cylinder is flooded, the light stuff (air) is lost and replaced by the same type of matter that is displaced (water). The buoyancy of the volume that is flooded thus goes to zero.

What is "zero buoyancy"?

The object was neutrally buoyant because it weighed exactly the same as the volume of water it displaced. Once water was introduced into the cylinder instead of of air, the scooter became negatively buoyant because it now weighs more than the volume of water it displaces.That volume has not changed, though.

I have a scooter that is beautifully neutrally buoyant in fresh water. When I take it to salt water, I add a very small amount of lead inside it. It now weighs more than it did, but it is still neutrally buoyant because the salt water weighs the same amount more. If I took that scooter back to fresh water with those weights still in it, it would sink, and it would sink surprisingly quickly.

 

[Jiminy]

I would think completely flooded or nose off it would be 20lbs or so negative because of batteries and motor but i dont know my maths is weak


Edit

looks like if this math is correct a 450lb cast iron cannon weighs 180lbs less in sea water

so weight in sea water would be 270 which is a 40 percent reduction in weight

Weight of an object in water

 

[Dominik_E]

boulderjohn, "zero buoyancy" means no force antiparallel to gravity because of displacement of surrounding medium.

I do not take objection to your statements regarding an imploding or flooding scooter loosing buoyancy and thus becoming negative. That is completely correct! The only thing that is not correct is that it would make a difference whether the scooter imploded completely or flooded completely. Both cases result in exactly the same negativity.