PDA

View Full Version : Physics question

JohnnyGreen
2010-Oct-11, 06:28 AM
I am 19 years old and have not been in a physics class for about two years so forgive me I'm a little off on the science here.

My question is; the speed of sound in air is 1,236 km/h and, I believe the speed of sound in water is 5,335 km/h. I was wondering if hypothetically, we had an underwater vessel that was powerful enough it could reach the speed of sound in water, what would happen?

In air, if a craft were to travel faster than the speed of sound, there's a sonic boom created. From my understanding, it's from when the craft begins to move faster than the air pressure waves it creates ahead of it and those waves turn into one big air pressure wave, and creates a boom. That's a very rough explanation but that's beside the point.

What I'm wondering is would there be an equivalent sonic boom in water. Also if that's the case, from my understanding, wouldn't it be much more powerful of a boom seeing as it takes A LOT more energy to reach the speed of sound underwater? Could it even create a tidal wave if the vessel were large enough?

I was however, also thinking that perhaps if there was a boom, although it does take more energy to reach that speed, perhaps could the extra density associated with water be enough that it would actually weaken that boom, and make it almost non-existent?

I'm actually in college to become an audio engineer, however physics has always been a huge interest of mine. And this has been a question I've wondered about for years, but I've never really asked a scientific community such as yourself. So ya, what are your thoughts??

Ken G
2010-Oct-11, 08:57 AM
What I'm wondering is would there be an equivalent sonic boom in water. Also if that's the case, from my understanding, wouldn't it be much more powerful of a boom seeing as it takes A LOT more energy to reach the speed of sound underwater? Could it even create a tidal wave if the vessel were large enough? A sonic boom is not really a lot of energy in any one place, enough to rattle a window but that's all. Magnifying that by a factor like 10 is still a long way from a tidal wave. But I think there would be some underwater equivalent to a sonic boom. There might also be a lot of other things going on in liquid water that doesn't happen in air, it's denser and less compressible.

swampyankee
2010-Oct-11, 10:18 AM
There would be a sonic boom; signals can't propagate faster than sound in a given fluid (or solid). A sonic boom is a result of this. The properties of a sonic boom in water is a difficult problem, as water has a much more complex equation of state than does air at normal atmospheric conditions. I suspect (which means I haven't done the math) there will be phase changes around a sonic boom in water.

grapes
2010-Oct-11, 10:32 AM
A sonic boom is not really a lot of energy in any one place, enough to rattle a window but that's all. Magnifying that by a factor like 10 is still a long way from a tidal wave. But I think there would be some underwater equivalent to a sonic boom. There might also be a lot of other things going on in liquid water that doesn't happen in air, it's denser and less compressible.If you're close enough, it can break windows. :)

Depends upon the size of the vessel, surely. I remember one documentary, I think it was narrated by Elijah Woods, that showed a tidal wave generated by an interplanetary vessel plowing into the Atlantic.

Ken G
2010-Oct-11, 10:48 AM
Depends upon the size of the vessel, surely. I remember one documentary, I think it was narrated by Elijah Woods, that showed a tidal wave generated by an interplanetary vessel plowing into the Atlantic.Sure, the total energy would depend on speed and size. I was imagining an airplane size at a speed of Mach 1.1 or some such thing, that sounded like what the OP meant. But any way you slice it, you can't get more energy out of the tidal wave than you put into the vessel, and a tidal wave has a whole lot of energy!

dgavin
2010-Oct-11, 04:48 PM
Actually, once a vessel hits certain speeds in water no matter it's shape, it will cause cavitation behind it. The faster the vessle the more cavitation, which attempts to pull the vessle back towards the cavity. Not sure of the math here, as i'm no fluid hydrodynamic expert, but my best guess is it would take as much enegy to break the speed of sound in water, as it would to accelerate a ship in space to about 10% light speed.

caveman1917
2010-Oct-11, 05:16 PM
Actually, once a vessel hits certain speeds in water no matter it's shape, it will cause cavitation behind it. The faster the vessle the more cavitation, which attempts to pull the vessle back towards the cavity. Not sure of the math here, as i'm no fluid hydrodynamic expert, but my best guess is it would take as much enegy to break the speed of sound in water, as it would to accelerate a ship in space to about 10% light speed.

That could be quite a tidal wave then, if a decent percentage of that energy is released in the sonic boom

trinitree88
2010-Oct-11, 06:40 PM
The sinking of the Kursk was by a torpedo alleged to be rocket-powered and going ~ 200 m.p.h. underwater, which is way subsonic but probably as fast a manmade object as you'll get. They lost control and it circled back to home. Doh. :doh::naughty::shifty:

see:http://en.wikipedia.org/wiki/Russian_submarine_Kursk_explosion

Hornblower
2010-Oct-11, 11:21 PM
The sinking of the Kursk was by a torpedo alleged to be rocket-powered and going ~ 200 m.p.h. underwater, which is way subsonic but probably as fast a manmade object as you'll get. They lost control and it circled back to home. Doh. :doh::naughty::shifty:

see:http://en.wikipedia.org/wiki/Russian_submarine_Kursk_explosion

That is not what the Wiki article to which you linked reports. It appears that a defective torpedo in one of the tubes suffered a fuel explosion, and in the resulting fire several more torpedoes "cooked off" in a massive explosion. I saw nothing about a torpedo circling at high speed.

Grashtel
2010-Oct-11, 11:59 PM
Actually, once a vessel hits certain speeds in water no matter it's shape, it will cause cavitation behind it. The faster the vessle the more cavitation, which attempts to pull the vessle back towards the cavity. Not sure of the math here, as i'm no fluid hydrodynamic expert, but my best guess is it would take as much enegy to break the speed of sound in water, as it would to accelerate a ship in space to about 10% light speed.
Cavitation isn't necessarily a bad thing. Supercavitating (http://en.wikipedia.org/wiki/Supercavitation) designs make use of it to dramatically reduce the amount of surface area in contact with the water allowing higher speeds, for example the Russian Shkval torpedo makes use it to reach speeds upwards of 200 knots (370km/h).

Ken G
2010-Oct-12, 06:26 PM
That is not what the Wiki article to which you linked reports. It appears that a defective torpedo in one of the tubes suffered a fuel explosion, and in the resulting fire several more torpedoes "cooked off" in a massive explosion. I saw nothing about a torpedo circling at high speed.What's more, a torpedo in most situations would have a safety that makes it not detonate if it is close to its source. I saw "The Hunt for Red October."