# Buoyancy, neutral or not?

1. Jan 15, 2014

### georgejeff

I believe that I am making this a bit harder than it should be.

An object is weighted so that it "hoovers" 5 feet underwater. It is neutrally buoyant. A diver drags the object even deeper without adding weight, she releases the object at a deepth of 100ft. Will the object stay put, rise up, or sink?

What do you think?

2. Jan 15, 2014

### Staff: Mentor

The question is, "what do you think?" and why.

3. Jan 15, 2014

### georgejeff

I think that buoyancy is not related to depth, yes the pressure on the object will increase overall at greater depth. However, the differential pressure (upward force - downward force) will remain unaffected. The weight difference is insignificant from the law of universal gravitation, furthermore, if we are talking about depths of less than say 4km, the density of the water in not a factor.

What do I think? I think that if the object "hovers" at five feet and we take it to a depth of 100 feet, it will "hover" at that depth as well. Is my logic sound?

4. Jan 15, 2014

### sophiecentaur

What is one major effect of great pressure? What will change? How will that affect the buoyancy?

(Dammit - why can't they just tell me the answer!!!!!)

5. Jan 15, 2014

### Microwire

I don't know that much about bouancy and water density but I've always beleived that the object would float up to or around to the 5ft mark.

If you were to test this it would depend greatly on what the object is made from as the preassue would compress the air inside and create a more dense object.

6. Jan 15, 2014

### sophiecentaur

An passive object that is floating, neutral at 5ft is actually unstable. It will go right up or right down, after a small perturbation. Fish and submarines need to adjust their buoyancy constantly to stay at a given depth. (See Fish Swim Bladder)

7. Jan 15, 2014

### Staff: Mentor

If water were incompressible and the object were incompressible, it would hover at 100 ft just like at 5 ft. Assuming that the object actually is incompressible, the water will compress slightly with depth. So, at depth, the weight of the water displaced by the object (a constant volume) will be greater. This would cause the object to float back up to the 5 ft depth, where it would hover again. To answer this question properly, however, you would have to know the change in volume of the object with pressure.

8. Jan 15, 2014

### Microwire

Submarines are what I was thinking about, they have their balast which is adjusted depending on what depth they want it's not just set and then taken to any depth.

9. Jan 16, 2014

### rude man

Basically the answer to your first question is that it will hover at 100 ft same as at 5 ft.

A submarine has to be able to dive and rise on command. It can do so by increasing or decreasing the ballast. But when it's done changing depth the amount of ballast is essentially the same at all depths.

It's an interesting transfer function: the system is basically an open-loop integrator with two inputs: a command depth, and a depth pressure sensor subtracting the command. The integrator (flow of ballast) stops when the two cancel each other and the desired depth is reached. The rate of submersion is controlled by the gain of the integrator (intake valve setting).

The dynamic system response can be improved by a PID controller (advanced subject!).

10. Jan 16, 2014

Yes.