# How can people float or sink in the water with no volume changed?

1. Jul 2, 2014

### Frigorifico

I am no english native speaker, sorry if something is a little confuse.

I was swimming, and then I realized that when I let the air go our of my lungs I sink. I though this might be because of changes in the volume of my body while holding air or letting it go. So I hold as much air as I could and also make my stomach as small as I could with my muscles, and I realized that I was floating independently of the shape of my stomach, then I let air go and I sink, independently of the shape of my stomach. I did this many times to be sure.

My question is, how does this is explained by buoyancy?, because it seems that the volume of my body had nothing to do if I floated or not, and I know that air is lighter than water but it only makes my heavier without expanding considerably my volume.

So, how can I possibly float?

2. Jul 2, 2014

### Staff: Mentor

You cannot make both your stomach region and your chest as small as possible with a lot of air in your lungs. Reducing the volume of one will just increase the volume of the other (to a good approximation).

3. Jul 2, 2014

### CWatters

Your lungs aren't strong enough to compress air significantly - so your volume is not constant when you breath in and out.

Think of your body like a balloon. You squeeze it in one place and it pops out in another. If you try and compress your stomach you normally end up expanding the chest not compressing the air in your lungs.

4. Jul 2, 2014

### SteamKing

Staff Emeritus
Just like any other object which floats. The overall density of your body is less than the density of the water you are in. This is partially due to internal cavities in your body, like your lungs, stomach, and various other passages. As long as your body displaces a weight of water equal to your body weight, Archimedes takes over and you are floating.

5. Jul 3, 2014

### sophiecentaur

A free diver is basically unstable. once the air in the lungs is compressed by the water above, the diver will sink faster and faster. A SCUBA diver can adjust the lung volume with air from the bottle and control his depth without swimming. Ascending without breathing out is very dodgy as your lungs can burst. One enormous long breath out can feel very odd as you are swimming upwards from depth.
Many fish have a swim bladder which contains a bubble of air. They use muscles to compress this bubble and this adjusts their depth with much less expenditure of energy than swimming would.
It may be of interest to know that Fish Finders (sonar) use the reflections off the swim bladders to detect the fish.

6. Jul 3, 2014

### Frigorifico

Sophicentaur referring to the examples of the divers, while I can believe that the lungs change in volume and weight significantly, enough to explain buoyancy, they are inside the body, and it doesn't seem that they change the volume of the body significantly. Is like if I have a bag that because of its volume and weight it can't float in water, and then I add two balloons full of hydrogen inside it without changing the volume of the bag, if anything the bag is heavier now and still shouldn't float. That's how I understand the body in this situation, and I know this point of view is wrong, but I don't know which is the correct one.

7. Jul 4, 2014

### sophiecentaur

Your body is not a rigid container. As the external pressure changes, it will change the volume of the most compressible part - the air in the lungs. You are pretty much like a balloon. A submarine, otoh, has a rigid hull and the living areas can be maintained at surface pressure.

8. Jul 7, 2014

### Frigorifico

All right, I understand, but it is hard to believe that such small changes in volume can be so significative, do you know of any experiment that illustrates this?. I tried with a balloon in a water tank to which I added air with a straw, but I lack precision

9. Jul 7, 2014

### olivermsun

Try using a rigid container or bottle. See if you can get it close to neutral buoyant by changing the amount of air inside the bottle. Now once you are neutral, try changing the ratio of air vs. water by small amounts

10. Jul 7, 2014

### sophiecentaur

Experiment on yourself (assuming you can swim and will be taking the usual safety precautions).Get into a swimming pool and have just enough air in your lungs to float. Breath out a very small amount - equivalent to compressing the air and reducing the total volume in your lungs and, hence, your total displacement. You will start to sink. Alternatively, try snorkelling and duck-diving down two or three metres. You will find that your buoyancy becomes neutral, then negative. It is very restful to lay on the sea bed with no effort because you don't need to keep swimming down.

Try the Cartesian diver experiment.

11. Jul 7, 2014

### BruceW

yeah, another example is when divers use bcd (buoyancy control devices). it is a jacket that can fill up with air from the tank. So, in other words, you can increase your volume, which increases your buoyancy. When the jacket has no air in it, the diver will immediately sink (since he has a massive scuba tank on his back, and possibly several weights too). But if he fills the jacket with air, he will shoot up to the surface at ridiculous speed. And this is not a great volume of air in the jacket. It really doesn't take much. It is because the density of water is surprisingly great. And air has practically zero density in comparison. So it doesn't take much air to lift up some heavy weights.

12. Jul 7, 2014

### Staff: Mentor

The volume changes are not that small: human lung capacity is about 6 liters, so that's an equivalent amount of water displaced and buoyancy. When holding your breath and swimming down 5 meters, you lose 2 liters of that buoyancy.

13. Jul 7, 2014

### sophiecentaur

You only need 0.1mcube (1 litre) of water displaced to produce an upthrust of 10N. But it's a runaway situation because the air will either expand or contract as the depth changes and the upthrust changes.

14. Jul 7, 2014

### sophiecentaur

Yes, you can actually feel your chest compress as you go down. At 10m depth, the lung capacity has halved. One of the nice bits of swimming back upwards is that you feel your lungs filling up - so much so that you can breathe out (gently) over the final metre or so.