beech ball buoyancy

[raptor10]

Buoyancy is the upward force acting on an object in a fluid. Okay so where does it come from?

It comes about because the downward force on the beach ball (from the pressure of the water above it) is smaller than the upward force on the beach ball (from the pressure of the water below it), because pressure increases with depth.

So just like a plane with High Pressure on bottom and Low Pressure on top it gets lift. Thats Buoyancy.

 

[eddie]

And that isn't what he said.

Dammit, my bad.

 

[Conaway]

I had this stupid question on my ASTB too.

 

[phrogpilot73]

I had this stupid question on my ASTB too.

I don't remember having that crappy worded question on mine. I'm with Brett - the study guide is wrong. Best to answer every question to the best of your ability vice what the study guide says... UNLESS you are clueless when it comes to Spatial Aperception...

 

[raptor10]

No the study guide is correct, think about it this way: If the beach ball was easy to hold at depth then it is at the surface, (like say oh a TV) then it would sink because its natural state would be to sink.

 

[HercDriver]

Thull: I hope we can get a good answer for you...this conundrum will come up quite often once you are commissioned.:icon_tong

 

[thull]

^^I hope so! or is this similar to a situation at OCS where you are made to fail no matter what and it's your reaction they're looking for? Do you get up and walk out of the ASTB in a fluster, or fill in something you know is wrong...and move on?

Buoyancy is the upward force acting on an object in a fluid. Okay so where does it come from?

It comes about because the downward force on the beach ball (from the pressure of the water above it) is smaller than the upward force on the beach ball (from the pressure of the water below it), because pressure increases with depth.

So just like a plane with High Pressure on bottom and Low Pressure on top it gets lift. Thats Buoyancy.

true, but the question was if the ball was "near the surface" meaning still completely submerged, vs "deep down"...so my conclusion is that what they meant was "near the surface" meaning ON the surface, in which case it would be just an extremely poorly worded question..

 

[AVDAD]

Completely out of my realm so I'll do what a politican might do and answer with a question.

If a person were to attach a scale like you might weigh a fish with, and begin to pull downward on the beach ball as it floats on the surface what would be noted as the ball went form floating to completely submerged?

Mid taking finals today so I won't bother him but I can wait to run this past him when he gets home tomorrow. I'm excited, I might actually learn something today.

My uneducated swag is that it seems more buoyant until it's under the surface. Then the surface of the ball is being affected around the complete vessel.

AvDad

 

[larbear]

No the study guide is correct, think about it this way: If the beach ball was easy to hold at depth then it is at the surface, (like say oh a TV) then it would sink because its natural state would be to sink.

OK you were right about the source of buoyancy. The book answer is still wrong though. The reason the ball has the same buoyancy at any submerged depth is because the pressure differential between the top and bottom of the ball is constant with depth. Remember P=Rho*g*h? Well the buoyant force is F = (P_bottom - P_top)*area = (h_bottom - h_top)*Rho*g*area. Since (h_top - h_bottom), Rho, g and area are constant (assuming the ball holds its size), the buoyant force is constant. This is a simplified form for a box-shaped ball, but it would still be true for a ball of any shape.

OK so the sign of the buoyant force might be mixed up a little in the middle, but that is small detail, so saith the spider.

 

[webmaster]

test

 

[raptor10]

true, but the question was if the ball was "near the surface" meaning still completely submerged, vs "deep down"...so my conclusion is that what they meant was "near the surface" meaning ON the surface, in which case it would be just an extremely poorly worded question..

Buoyancy exists for any fluid... including air. At the surface of the water it doesn't have enough buoyancy to just float away (unless it was filled with helium which in that case would just magnify the previous scenario)

OK you were right...

****The reason the ball has the same buoyancy at any submerged depth is because the pressure differential between the top and bottom of the ball is constant with depth. ****

Yes, and this is where the hidden genius of the question comes into play, The question is not "when is the ball more buoyant," but "when does the ball SEEM more buoyant." Buoyancy is constant. (yes even in a submarine)

However to the average person watching the beach ball shoot up from under the water like some mini ICBM, it would SEEM that the ball is less buoyant at the surface because it is now at rest, when in fact it is only lying there because it is more dense then the air which it is now displacing.

Its a craftily designed question, not a poorly designed one...

 

[Sly1978]

I think you're trying to "engineer-think" this one. Let me put it like a business major (AKA Barney style).

Take a beach ball, put it at the surface of the pool and then try to force it to go under completely. Now keep pushing it. I don't know about engineering, but I do know that trying to get a regular beach ball (or any floating object really) more than a foot or two under can be a real sonofabitch. Therefore, I would answer "B" to both questions. I can't remember if I saw anything like it on the ASTB.

 

[larbear]

I think you're trying to "engineer-think" this one. Let me put it like a business major (AKA Barney style).

Take a beach ball, put it at the surface of the pool and then try to force it to go under completely. Now keep pushing it. I don't know about engineering, but I do know that trying to get a regular beach ball (or any floating object really) more than a foot or two under can be a real sonofabitch. Therefore, I would answer "B" to both questions. I can't remember if I saw anything like it on the ASTB.

Very true. This is because you are presumably trying to push the ball down while you are in the water yourself. It only seems hard to push the ball very far down because you are basically floating near the surface and your arms have a limit to how far you can push the ball below your body. Try it without entering the water yourself and with longer arms. It will seem way easier and answer "B" might not be correct anymore.

 

[thull]

Buoyancy exists for any fluid... including air. At the surface of the water it doesn't have enough buoyancy to just float away (unless it was filled with helium which in that case would just magnify the previous scenario)
Yes, and this is where the hidden genius of the question comes into play, The question is not "when is the ball more buoyant," but "when does the ball SEEM more buoyant." Buoyancy is constant. (yes even in a submarine)

However to the average person watching the beach ball shoot up from under the water like some mini ICBM, it would SEEM that the ball is less buoyant at the surface because it is now at rest, when in fact it is only lying there because it is more dense then the air which it is now displacing.

Its a craftily designed question, not a poorly designed one...

However...the question was not "AT" the surface, but rather "NEAR" the surface. If the beach ball is tied to a rope and you're at the bottom of a deep swimming pool with cinder blocks attached to your feet, holding it so that it is totally submerged an inch under water would be just as hard as holding it 15 feet below the water.

 

[Brett327]

This is turning in to the next installment of "Watch the group of engineering monkeys try to fvck a football." It's a bad question and it won't get any better with a bunch of slide rule toting morons trying to over-think the problem.

Brett