Wikipedia:Reference desk/Archives/Science/2024 July 28
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July 28
editWhat is the geological composition of Lascaux?
editI am an artist currently working on a project themed around prehistory and I'd like to know about the geological composition of Lascaux (as well as other early human settlements and pre-human geography in general), so I can better represent it. Additionally, is there any sort of database I can use for these purposes? Aedenuniverse (talk) 01:20, 28 July 2024 (UTC)
- See karst. Not sure that helps much. Sean.hoyland (talk) 06:05, 28 July 2024 (UTC)
- Specifically, it is reported as being calcarenite from the upper Coniacian. Sean.hoyland (talk) 06:10, 28 July 2024 (UTC)
- The function of Lascaux is a topic of academic debate, but it was not a settlement. It, and similar caves, are not suitable for serving as dwellings. Human groups of that time lived in tents or the open air, and may occasionally have found shelter in much shallower caves. --Lambiam 12:02, 28 July 2024 (UTC)
Conserving the kinetic energy, without conserving the momentum. Possible?
editThere being no external forces, inelastic collisions conserve the whole system's momentum, without conserving the whole system's kinetic energy.
What about the opposite physical process? I.E. is there any physical process, e.g. with external forces, which conserves the whole system's kinetic energy, without conserving the whole system's momentum? HOTmag (talk) 07:27, 28 July 2024 (UTC)
Resolved- I've just thought about it:
- The whole system is: a single elastic body.
- The physical process is as follows: the single elastic body collides with an elastic wall, being the external force.
- Result: the single elastic body is pushed back to the oppposite direction, with the same speed as before the elastic collision took place.
- Therefore: the elastic body's kinetic energy is conserved, yet the elastic body's momentum is not. QED. HOTmag (talk) 10:05, 28 July 2024 (UTC)
- Nah, you've ignored newton 3.
QEDGreglocock (talk) 23:47, 28 July 2024 (UTC)- Why do you think I ignored it? I think I didn't. HOTmag (talk) 06:47, 29 July 2024 (UTC)
- My second thought is to underline 3 words in my response below. Ignoring what is happening by calling one involved object "the whole system" is disingenuous. Philvoids (talk) 18:21, 29 July 2024 (UTC)
- The decision of what the "whole system" is, depends on our choice. Here is a typical example: If two perfectly elastic bodies collide with each other, we can choose, whether to call the two-body system: "the whole system", in which case no external force is involved - hence the momentum is conserved in our "whole system" chosen, or to call one colliding body alone "the whole system", in which case the other body exerts an external force on our "whole system" chosen - hence the momentum is not conserved in our "whole system" chosen. This is how all of physics works, with no exceptions. HOTmag (talk) 20:39, 29 July 2024 (UTC)
- The wall will gain some momentum. In a multi body collision the centre of gravity of the system remains at constant velocity (N1) in the absence of external forces. You are 100% wrong, and are just making up random explanations to cover up your lack of understanding. I'll be ignoring you from now on. Greglocock (talk) 23:54, 29 July 2024 (UTC)
- I'm referring to a very specific reference frame, which is the wall's reference frame, in which every observer referring to the elastic body as "the whole system" attributes no change to the wall's momentum.
- I'm bad at analyzing personal comments, so I'm letting the users decide who is right and who is wrong.
- HOTmag (talk) 09:18, 30 July 2024 (UTC)
- If you're analysing a collision, you can't just pick one of the things that's colliding and call it the whole system. If it was the whole system, there would be nothing else for it to collide with. "The whole system" is not a reference frame. It's the whole set of objects, by definition; you can't pick and choose which objects it includes. AlmostReadytoFly (talk) 09:57, 30 July 2024 (UTC)
- A side note: I've never claimed that "the whole system" is a reference frame.
- As to your main response: Can you give any example of a two-body system, for which our referring to one of them as the "whole system" may contradict the laws of physics? AFAIK, there is no example of this kind. Recommendation: before you try to think about such an example, take another look at the example I've already given in my previous-previous response (i.e. the one beginning with "The decision"). HOTmag (talk) 10:35, 30 July 2024 (UTC)
- If you have a two-body system, the two bodies are the system. If you say you have a one body system, then say that that body collides with something else (e.g. a wall), you're contradicting yourself. AlmostReadytoFly (talk) 11:30, 30 July 2024 (UTC)
- Maybe the expression "the whole system" confuses you. I can replace it by the expression "the sub-system chosen", in which case no contradiction follows, even according to your attitude.
- One contradicts oneself when one says "x" and then says "not x". My analysis is not the case, even when the expression "whole system" is used, but I'm changing it for you, to avoid confusion. HOTmag (talk) 12:32, 30 July 2024 (UTC)
- "When I use a word," Humpty Dumpty said in rather a scornful tone, "it means just what I choose it to mean——neither more nor less."
- If you just want a physical process where a body keeps its kinetic energy but not its momentum, consider a body in a circular orbit. AlmostReadytoFly (talk) 13:00, 30 July 2024 (UTC)
- Humpty Dumpty didn't care if the way he spoke could confuse others, but I do care, and that's why I changed the expression "whole-system" to "sub-system".
- Yes, also the body you suggest can be chosen as the sub-system. But also the body I've suggested can. HOTmag (talk) 17:56, 30 July 2024 (UTC)
- Your Question: is there any physical process, e.g. with external forces, which conserves ... kinetic energy, without conserving ... momentum?
- Answer: Yes, a circular orbit. AlmostReadytoFly (talk) 18:24, 30 July 2024 (UTC)
- I've already approved your answer, in my previous response, so I wonder why you had to repeat the same answer. I only added that also my answer (that preceded yours) was correct. HOTmag (talk) 18:59, 30 July 2024 (UTC)
- Maybe the expression "the whole system" confuses you. I can replace it by the expression "the sub-system chosen", in which case no contradiction follows, even according to your attitude.
- If you have a two-body system, the two bodies are the system. If you say you have a one body system, then say that that body collides with something else (e.g. a wall), you're contradicting yourself. AlmostReadytoFly (talk) 11:30, 30 July 2024 (UTC)
- A side note: I've never claimed that "the whole system" is a reference frame.
- If you're analysing a collision, you can't just pick one of the things that's colliding and call it the whole system. If it was the whole system, there would be nothing else for it to collide with. "The whole system" is not a reference frame. It's the whole set of objects, by definition; you can't pick and choose which objects it includes. AlmostReadytoFly (talk) 09:57, 30 July 2024 (UTC)
- I'm referring to a very specific reference frame, which is the wall's reference frame, in which every observer referring to the elastic body as "the whole system" attributes no change to the wall's momentum.
- The wall will gain some momentum. In a multi body collision the centre of gravity of the system remains at constant velocity (N1) in the absence of external forces. You are 100% wrong, and are just making up random explanations to cover up your lack of understanding. I'll be ignoring you from now on. Greglocock (talk) 23:54, 29 July 2024 (UTC)
- The decision of what the "whole system" is, depends on our choice. Here is a typical example: If two perfectly elastic bodies collide with each other, we can choose, whether to call the two-body system: "the whole system", in which case no external force is involved - hence the momentum is conserved in our "whole system" chosen, or to call one colliding body alone "the whole system", in which case the other body exerts an external force on our "whole system" chosen - hence the momentum is not conserved in our "whole system" chosen. This is how all of physics works, with no exceptions. HOTmag (talk) 20:39, 29 July 2024 (UTC)
- My second thought is to underline 3 words in my response below. Ignoring what is happening by calling one involved object "the whole system" is disingenuous. Philvoids (talk) 18:21, 29 July 2024 (UTC)
- Why do you think I ignored it? I think I didn't. HOTmag (talk) 06:47, 29 July 2024 (UTC)
- Nah, you've ignored newton 3.
- (ec)
- All types of collision (inelastic or elastic) conserve momentum. Total kinetic energy would be conserved (meaning no release of sound or heat) only in an impractical perfectly elastic collision. To identify a process that conserves a system's kinetic energy but may be affected by external forces, one needs firstly to clarify whether the system shall qualify as an Isolated system where thermodynamic laws apply. The Second law of thermodynamics observes that the entropy of isolated systems left to spontaneous evolution cannot decrease, as they always tend toward a state of thermodynamic equilibrium where the entropy is highest at the given internal energy. Philvoids (talk) 10:10, 28 July 2024 (UTC)
