# B Upthrust in a liquid

1. Dec 29, 2016

I have a question about upthrust. I have studied that if an object is placed in a fluid and if the upthrust is not equal to the weight of the object even after the object is fully immersed in the fluid, it sinks. My question is that is it possible for an object immersed in water to keep sinking until it reaches a particular level or depth underwater i.e when upthrust equalizes the weight of the object.I think upthrust increases with depth so a point should come when the object should stop sinking. Can someone please help me out here.

2. Dec 29, 2016

### Orodruin

Staff Emeritus
Water is not very compressible and so the buoyant force on an object of fixed size does not change much with depth. On the contrary, if an object is more compressible than water - it will displace less water at larger pressures and the buoyant force will decrease with depth.

3. Dec 29, 2016

### A.T.

If the density of the fluid inreases with depth.

4. Dec 29, 2016

So what you are tryin to say is that upthrust doesn't change as the object sinks deeper. only an increase in density of the fluid as the object sinks can result in an increase in upthrust

5. Dec 29, 2016

### A.T.

Yes.

6. Dec 29, 2016

Thanks alot☺

7. Jan 1, 2017

### Baluncore

Upthrust = buoyancy. A submarine is more compressible than water. As hydrostatic pressure increases with depth the submarine reduces volume faster than does an equal mass of water. As a submarine goes deeper the buoyancy reduces and so it sinks faster. There is no stable depth, only the sea floor.

The density of water changes with temperature and salinity. If there is a stable layer of more dense water below, then a submarine can remain sitting at that particular depth without a need to continuously adjust bouyancy.

8. Jan 1, 2017

### Orodruin

Staff Emeritus
This is also true for a diver who is using to much weights. Buoyancy is usually controlled by letting air into and out of a vest you are wearing. If you wear the correct amount of weights, you should have neutral buoyancy with an empty vest and your lungs half full - letting you control you floatability with your breath. If you need to fill the vest to maintain neutral buoyancy the air in it will compress and expand as you go down/up - resulting in an unstable system that can be quite dangerous to the diver (ascending too fast is not healthy).

A diver with too little weight will always need to apply a swimming force to stay submerged.