B Why do things burn up in the atmosphere but not in the ocean?

[Feynstein100]

As the title suggests, it just occurred to me that things don't burn up when entering the ocean as opposed to when entering the atmosphere. This seems counterintuitive because the explanation behind atmospheric burnup is usually attributed to friction. In that case, water should provide even more friction and thus make the object burn up even faster and yet that doesn't happen. So why not? Could someone please help me out?

 

[kuruman]

It seems to me that the water will put out the fire before it started. Have you ever given consideration to the mechanism(s) whereby water is used to put out fires?

 

[PeroK]

First, an object has to penetrate the atmosphere.

Second, the atmosphere gets gradually denser. Whereas, an impact on a body of water would generate a large sudden impulse.

 

[Feynstein100]

It seems to me that the water will put out the fire before it started. Have you ever given consideration to the mechanism(s) whereby water is used to put out fires?

How insightful! I hadn't thought of that at all. Thank you for enlightening an ignoramus like myself with your profound grasp of physics. I am truly blessed to be in the company of such extraordinary minds.

 

[A.T.]

....because the explanation behind atmospheric burnup is usually attributed to friction...

For things like meteors entering the atmosphere the heating comes mostly from compressing the gases in front of them:
https://en.wikipedia.org/wiki/Atmospheric_entry

Since liquids cannot be compressed much, that mechanism plays less of role there. The other mechanism is friction, which exist in water too, but since water can also absorb a lot of heat and transport it away, it's not that easy to get warm this way.

 

[kuruman]

How insightful! I hadn't thought of that at all. Thank you for enlightening an ignoramus like myself with your profound grasp of physics. I am truly blessed to be in the company of such extraordinary minds.

You are welcome. Your sincere appreciation of my efforts encourages me to state two observations to enrich your enlightenment.
(a) The ocean contains substantial amounts of coolant water which will carry away any heat generated by friction before the temperature of the object rises to the point of initiating combustion.
(b) Even if it were possible to initiate combustion, the ocean does not contain enough free molecular oxygen to sustain it.

 

[256bits]

Actually, your first statement held all this explanation in truncated form.
It's always good to have 2way communication
Well done I say.

 

[jbriggs444]

As the title suggests, it just occurred to me that things don't burn up when entering the ocean as opposed to when entering the atmosphere. This seems counterintuitive because the explanation behind atmospheric burnup is usually attributed to friction. In that case, water should provide even more friction and thus make the object burn up even faster and yet that doesn't happen.

Wait a bit. Do we actually know that meteoroids impacting the ocean fail to vaporize?

https://en.wikipedia.org/wiki/Eltanin_impact

 

[russ_watters]

As the title suggests, it just occurred to me that things don't burn up when entering the ocean as opposed to when entering the atmosphere. This seems counterintuitive because the explanation behind atmospheric burnup is usually attributed to friction. In that case, water should provide even more friction and thus make the object burn up even faster and yet that doesn't happen. So why not? Could someone please help me out?

Things don't hit the ocean at 17,000 mph. Or if they do it's a lot worse than just heating up.

 

[Baluncore]

The forces that result when a supersonic object collides with water are so great, that water enters every crack or fissure in the surface, which shatters the object, and vaporises water that is displaced.

Water is used for cooling the reaction when fighting small fires. The steam produced will also displace oxygen from the fuel. However, at great enough compression temperatures the water is disassociated, and so the hydrogen and oxygen do react chemically with the object. You will not see that happening because you cannot be there at the time.

https://www.cws.com/en/fire-safety/news/extinguishing-metal-fires-how
"The ignition temperatures of metals such as aluminum, lithium, titanium, sodium or magnesium are relatively high at more than 500°C. A metal fire is therefore rare, but if it does occur, fighting it is complex. Accordingly, a metal fire is one of the most challenging fires, as conventional extinguishing agents (e.g. water) are ineffective or even lead to dangerous oxyhydrogen explosions. The reason for this is that the temperatures are immensely high at more than 2,000 °C. As a result, water would split into hydrogen and oxygen in the course of firefighting.
Remember: Water must not be used as an extinguishing agent for a metal fire! Foam or CO2 extinguishers are also not recommended.

Metal fires should be extinguished with special metal fire extinguishers. They have been developed for fire class D, i.e. fires involving titanium, aluminum or magnesium, and contain a special extinguishing powder. This prevents the chemical reaction in the event of a metal fire."

 

[Feynstein100]

You are welcome. Your sincere appreciation of my efforts encourages me to state two observations to enrich your enlightenment.
(a) The ocean contains substantial amounts of coolant water which will carry away any heat generated by friction before the temperature of the object rises to the point of initiating combustion.
(b) Even if it were possible to initiate combustion, the ocean does not contain enough free molecular oxygen to sustain it.

Since you are so learned, I'd like to ask you a further question. On one end we have re-entry in air which causes objects to heat up and melt/vaporize. On the other hand we have re-entry in water which causes them to mostly just slow down. But both are fluids. The question is merely that of phase and density. So if we were to make air denser and denser without turning it into a liquid, it follows that at some density, objects entering it will no longer burn up but simply slow down, just as they do in water. I ask you at what density this tipping point lies and how it can be calculated. An enlightened one such as yourself shouldn't have any problems with a trivial scenario like this.

 

[Feynstein100]

Wait a bit. Do we actually know that meteoroids impacting the ocean fail to vaporize?

https://en.wikipedia.org/wiki/Eltanin_impact

Hmm good point. I guess gases don't have "surfaces" to provide impact like liquids do. At that point, the liquid acts more like a solid than like a gas since vaporization upon impact is what happens when solids collide with other solids as far as I know. I'm using the Barringer Crater in Arizona as reference here.
But the original question was regarding motion through a liquid, not instant vaporization upon impact. So while interesting, I don't think it's strictly relevant?

 

[jbriggs444]

On one end we have re-entry in air which causes objects to heat up and melt/vaporize. On the other hand we have re-entry in water which causes them to mostly just slow down.

Again, what makes you think that water just causes them to slow down with no other ill effects?

 

[jbriggs444]

Hmm good point. I guess gases don't have "surfaces" to provide impact like liquids do. At that point, the liquid acts more like a solid than like a gas since vaporization upon impact is what happens when solids collide with other solids as far as I know. I'm using the Barringer Crater in Arizona as reference here.
But the original question was regarding motion through a liquid, not instant vaporization upon impact. So while interesting, I don't think it's strictly relevant?

At sufficient collision energies, the distinction between solid, liquid and gas becomes irrelevant. Everything becomes a plasma or a supercritical fluid.

Our every day imagination thinks of rocks falling in water at modest rates, slowed by viscous friction and cooled gently by the water. Or of a bullet penetrating the water and leaving a cavitation wake a few meters long

An impact at 17 km/sec or more involves energies such that the energy required to melt or vaporize rock or the energy required to vaporize or chemically disassociate water are small in comparison. We should not expect our intuitions to be accurate guides to the resulting behavior.

 

[Feynstein100]

For things like meteors entering the atmosphere the heating comes mostly from compressing the gases in front of them:
https://en.wikipedia.org/wiki/Atmospheric_entry

Since liquids cannot be compressed much, that mechanism plays less of role there. The other mechanism is friction, which exist in water too, but since water can also absorb a lot of heat and transport it away, it's not that easy to get warm this way.

Thank you so much! This is exactly the explanation I was looking for. Why can't everyone be like you?