# If density of object is slightly greater than density of fluid?

1. Aug 12, 2014

### hongiddong

I know that if density of an object is equal to density of fluid, the object will be fully submerged hovering right underneath the fluid.

My questions related to this concept are:

1. If the density of the object is extremely slightly greater than the density of the fluid, will the object look as if it is suspended more under the fluid than at the horizon, but it is actually slowly accelerating to the bottom?

2. If my premise for 1. is wrong, how can an object be fully suspended in water and be lower than at the surface of the fluid without accelerating downward.

3. Lastly, if we were to have a scale underwater, would the apparent weight of an object with equal density to fluid density be zero?

Thanks Physics forums!

Last edited: Aug 12, 2014
2. Aug 12, 2014

### Orodruin

Staff Emeritus
1. It is unclear what you mean by "suspended more under the fluid than at the horizon". An object with greater density than the fluid will have a net acceleration down due to negative buoyancy. Now, if the object is moving, this net acceleration may be countered by the resistance of the fluid. This means that if you have two objects identical in shape, volume and surface condition, but of different density, the heavier one will have a higher terminal velocity.

3. Yes. The object would have neutral buoyancy.
(As a side note, as a scuba diver, you will try to aim for neutral buoyancy when your buoyancy control device is empty. If you do not find this sweetspot, the air in your BCD will expand when you go up - increasing your buoyancy - or be compressed when you go down. This represents a very unstable buoyancy and could result in uncontrolled ascents if you are not careful - such quick ascents may be outright dangerous. On another side note, my diving instructor had mixed feelings about trying to teach diving physics to a physicist ;))

3. Aug 12, 2014

### SteamKing

Staff Emeritus
'extremely slightly'? Which is it?

4. Aug 12, 2014

### hongiddong

Ok ok I see I see thank you Orodruin! By extremely slightly I meant slightly, SteamKing.

5. Aug 12, 2014

### Staff: Mentor

I think I can just answer what I think you mean:

In the real world, no object has a fixed buoyancy. For some objects it increases with depth and for others it decreases with depth.

If the net force is down for an object just below the surface, but buoyancy is increasing with depth, there is a stable depth that the object will sink to.

If the net force is down for an object just below the surface, but buoyancy is decreasing with depth, it will accelerate all the way to the bottom.

6. Aug 12, 2014

### Orodruin

Staff Emeritus
Or, if it is up at the bottom and decreasing with depth (so increasing as you go up), it will accelerate all the way to the surface - as in the case of the diver with air left in the BCD. This is the reason divers want to have just enough weights to be neutral without air in the BCD. If you put too much you will have stability problems, too little and you will not be able to get down (as you will be positive even with an empty BCD).

Edit: just as an aside, the object with buoyancy increasing as you go down will performed damped oscillations around the equilibrium point. Whether or not it overshoots the first time around depends on the amount of damping.

7. Aug 12, 2014

### olivermsun

Yup, that's what "waves" are.

8. Aug 13, 2014

### sophiecentaur

I had exactly the same problem with a parachuting instructor. He was Army Parachute Association and wanted us all to do it by numbers. He just couldn't handle technical enthusiasm in his students.