Bullet shot underwater where do the bubbles come from?

[ExNihilo]

Homework Statement

Hi,

A gun totally immersed underwater and shoot a bullet.

Q1. Would there be a trail of bubbles behind the bullet?

Q2. If there are bubbles, where do they come from?

Q3. If there are bubbles, would the sum of the volume of all bubbles be equal to the volume of the bullet?

Homework Equations

??

The Attempt at a Solution

I have no idea!


This is not a homework (I am way past school age). The reason I ask this question was actually come from swimming, where we are advised to make as few bubbles as possible. The "demo" to illustrate that principle was if you begin the arm stroke underwater then it is impossible to create bubbles. However, I remember having seen in movies gunshot underwater and there were a trail of bubbles. Overall I am totally confused by this bubbles affair.

Unfortunately, I cannot offer any explanation nor attempt of solution. I hope some one here can help me to give some simple explanations. Good enough answer is OK I don't need elaborate equations.

Thanks in advance for any help.

 

[zorro]

For bubbles formation during swimming, this might help http://books.google.com/books?id=fh...=onepage&q=arm stroke creates bubbles&f=false

 

[gneill]

Look up "Cavitation". There should be a very low pressure region directly behind the bullet as it moves through the water.

 

[ExNihilo]

Look up "Cavitation". There should be a very low pressure region directly behind the bullet as it moves through the water.

You are going to laugh. Several years ago, I had a discussion among swimmers. Anyone who swims will make bubbles. As I had no idea where these bubbles were created, I had suggested that it was cavitation. Thinking naively that the human arm is like a boat propeller but moving much slower and making less bubbles. I was not so sure b/c if it was cavitation it would probably puncture the skin. And indeed, a few swimmers laughed at me. Those who understand, suggested that the bubbles in swimming come from the air you grab above the surface which somehow is partially trapped in the water with the arm movement.

Today, funny enough, I just think back about this bubbles issue and wonder how they would be created if the movement begins from underwater. Will read more about the links you suggested. Hope I will fins some answers there. Thanks you all.

EDIT: Cavitation is likely the explanation.

http://en.wikipedia.org/wiki/Cavitation
Bullet in water:
Bullet Cavitation: http://www.youtube.com/watch?v=Pb2obCNdhUI&NR=1
Underwater Shooting:

 

[ExNihilo]

For bubbles formation during swimming, this might help http://books.google.com/books?id=fh...=onepage&q=arm stroke creates bubbles&f=false

This is a swimming technique book. And they all recommend smooth movement to create less bubbles (to catch the "hard" water). None of these books explain about the mechanism which causes these bubbles to appear. For good reason, because from the swimming standpoint, the physics of these bubbles is not important. All you need to know is the practice how to reduce them.

I just happened to be the guy who wonder about these weird questions. I remember things better when I understand how they works.

 

[zorro]

Well, apart from cavitation, bubbles are also formed due to the exothermic reaction of the gunpowder/other chemicals inside the bullet which release gases.

btw can you show a video where bubbles are created by a swimmer inside water? I don't think it is possible unless some air is trapped inside your clothes or your arms rotate as fast as a propeller

 

[MrRiggs]

Forgive me if my reply sounds ridiculous, I'm new here and my education consists of almost no academic qualifications whatsoever.

To my knowledge, the water that surrounds a fish contains dissolved oxygen at a ratio of about 5ml per litre, so maybe a sudden force of movement acting on an equal pressure of water would disrupt the equal spaces between gases, further causing them to collide and form bubbles?

The bond between Hydrogen and Oxygen atoms in water molecules can be broken by electricity, so in regards to swimmers, maybe a tiny charge builds during the kinetic movement of the body?

I don't know. I guess I have a lot to learn.
I've only just read the site rules. Apologies if everything I just said was 'crackpot' material.

 

[ExNihilo]

Well, apart from cavitation, bubbles are also formed due to the exothermic reaction of the gunpowder/other chemicals inside the bullet which release gases.

Good point, but in some video, you can see that there are bubbles in the trail of the bullet far away from the gun. Those are the bubbles that puzzled me.

btw can you show a video where bubbles are created by a swimmer inside water? I don't think it is possible unless some air is trapped inside your clothes or your arms rotate as fast as a propeller

When underwater, it's impossible for a swimmer to generate any bubble from the arms and legs movement. However on a normal swim, there are full of bubbles. Basically that was the reason I wondered how bubbles could be created when the movement started from underwater.

 

[ExNihilo]

Forgive me if my reply sounds ridiculous, I'm new here and my education consists of almost no academic qualifications whatsoever.

I am even more novice, just signed up yesterday. This forum is no joke. I was reminded by the forum admin that for this kind of question, I must move to the "Homework" section. BTW, if you know of other places where people could discuss about science, I would appreciate some links.

To my knowledge, the water that surrounds a fish contains dissolved oxygen at a ratio of about 5ml per litre, so maybe a sudden force of movement acting on an equal pressure of water would disrupt the equal spaces between gases, further causing them to collide and form bubbles?

The bond between Hydrogen and Oxygen atoms in water molecules can be broken by electricity, so in regards to swimmers, maybe a tiny charge builds during the kinetic movement of the body?

I don't know. I guess I have a lot to learn.
I've only just read the site rules. Apologies if everything I just said was 'crackpot' material.

It's better than no explanation at all. I take that as an explanation of how swimmers generate bubbles while swimming? If so, I don't think that the human movement could have enough energy to generate that much bubbles by this mechanism. Some novice swimmers make a huge number of bubbles. That you can even feel physically when you swim behind.

 

[JaredJames]

For the record:

When you move your arm underwater (not breaking the surface), no bubbles are created.

Cavitation, as above would be the main reason for something like this with a propeller or bullet.

 

[ExNihilo]

For the record:

When you move your arm underwater (not breaking the surface), no bubbles are created.

Thanks, noted. As in Post #1 and #8