Since molecules that move faster are hotter, why doesn’t ice melt if you throw it really fast?
By the same token, how can a cold wind blow? Shouldn’t it get hotter as it moves faster?
BZ- I hope you checked with the Mod’s, before you posted this…
It is a serious question, I was discussing with someone and realized I have no idea why throwing ice doesn’t make it melt. The only thing I could think of that it wouldn’t change the speed of the molecules relative to each other, which I am pretty sure is wrong.
If the mods want to remove it or close it they can.
If you put a chunk of ice in an insulated container and fired it out of a howitzer, it wouldn’t melt any faster than a stationary chunk of ice similarly insulated. It’s not the speed of the ice as a whole that counts, it’s the speed at which the molecules within the chunk move around relative to one another.
At a high enough speed, the friction of the passage through the atmosphere would cause a rise in teperature.
Well, I guess at any speed there would be some heating, but at low speeds it would be insignificant.
This is a joke, right?
Also, for the most part, wind doesn’t blow because of temperature; it blows because of pressure. There’s a low-pressure area way downwind, and a high-pressure area upwind, and the air just goes from the high-pressure area to the low-pressure area.
Throwing the ice isn’t going to add enough energy to melt it. It takes a lot more energy to melt something than it does to move it–I can push my car, for example…
You are getting confused between macro level movement and the movement of individual particles that make up a substance.
This is easy enought to do, as most people do not have direct experience with the behaviour of materials at the (sub)microscopic level.
What you should get from this is that the temperature of an individual particle is the same as its individual velocity (relative to its neighbors and surroundings) and that the temperature of a composite object is the average of the temperatures (motions) of its constituent atoms and molecules.
However, technically… if you did throw a snow ball fast enough, it would melt it. But that would be due to frictional or compression heating as it moved through the atmosphere.
Off topic…
Try to give BZ a break on these types of questions for a bit. I have set him a fairly difficult task and he is trying to accomplish it.
He is recognizing that he doesn’t have the background to really consider the stuff that interests him. He is working to rectify that. It is just new to him, but I congratulate him for making the effort to learn the science (physics) from the ground up so that he will be ready to consider more advanced topics in the hopefully not too distant future.
Hmmm. I guess I owe BZ an apology. I honestly didn’t think anyone could be serious with this question. I’m sorry.
Basically, BZ00000, you are misunderstanding what people mean when they talk about molecular motion as a manifestation of thermal energy.
When we say “molecular motion,” we mean “molecular vibration.”
The molecules in any substance warmer than absolute zero are always vibrating. Hotter molecules vibrate at a higher frequency and amplitude than cooler ones.
When you throw a chunck of ice you are simply accelerating it to an overall velocity, but you aren’t affecting molecular vibration at all.
Thus, no increase in stored thermal energy, no increase in temperature, and no melting.
Did that make sense?
Also, BZ… I have some further material that is extremely readable and clear on this exact topic if you are interested.
This would be taken from Feynman’s Lectures on Physics. I will be happy to forward key passages of this by email if you like. It describes molecular motion in solid’s, liquids, and gasses in an extremely illuminating way. He uses extremely revealing analogies that are easy to relate to, and then effectively points out the areas where the analogies fail so that you get a strong understanding easily, and is still quite accurate.
What did BZ get banned for?
Not that I need an obvious reason, just wondering if there is one…
ack… I didn’t notice he was banned… I have no idea.
I am quite curious myself though.
To be honest, I found him oddly amusing. Some of the stuff he posted was off the wall-truly it was, but other times I was very curious to see if he really felt that way…
Exgineer, i don’t think that’s correct. I believe the definition of temperature applies to the total kinetic energy of the molecules, which in a gas definitely includes the straight-line movement of the molecules as they’re bouncing off the walls of their container. That’s why pressure correlates with temperature. In the case of solids and liquids, most of the kinetic energy is vibrational, but that doesn’t make it the definition of temperature.
The reason why throwing a snowball doesn’t raise its temperature is simple. The kinetic energy of a mass is defined by its motion with respect to a frame of reference. The snowball is moving in your frame of reference, but not in its own frame of reference. The proper frame of reference for determining the molecular kinetic energy (and the temperature) of a mass is the center of that mass. Therefore, a mass has the same molecular kinetic energy when it’s moving as when it’s standing still.
I would say that nametag is very precisely correct and I thought about offering the same correction, but when a molecule is confined to a solid or liquid… vibration is a pretty “useful” way to look at it.
Whoops. I guess BZ asked one too many astonishingly silly questions. This one wasn’t that bad, honestly, but others…
Take-home lesson: if you’re learning from the ground up, ask ground-level questions. If you don’t understand the answers, don’t argue – ask a better question.
When you throw anything at all through the air there is an region directly in front of the object where the velocity of the air relative to the object is zero. This means that some air in front of the object has been brought from some different, initial velocity up to the same velocity as the object. So some energy has been imparted to that increment of the air. The pressure of that air has been raised because of this energy and the temperature as a consequence has also risen to the “stagnation temperature” for that particular gas (air) and speed.
The stagnation temperature for the speeds at which you can throw a piece of ice is so low that you really wouldn’t notice any increase in the melting rate. The flow of air past the object might increase the heat transfer into the ice and speed up melting that way, but all in all you probably won’t really notice much difference.
Now if you could throw the ice at Mach 1 it would turn to steam.
I think it might be the “coldfire” thread in the pit that did him in.
However, if the mods are interested in my opinion… while I did flat accuse him of trollery myself in the past. I have since changed that stance when I realized he really was just that painfully ignorant of many things that most people would just take for granted. Also, severe ignorance tends to handicap a persons ability to reason well as there is little to reason with.
I will also admit I may be somewhat partial on this now as I had adopted him as my pet project for “intellectual renewal”. It has been rough, but… he has been seriously trying and has learned a number of new things. This thread is a testament to his attempt to get through a pretty rough physics book I suggested as a good starting place.