# Thread: Some question of physics

1. 1)Why fire is hot?
2)Why ice is cold?
3)Why temperature on mountain is lower than the temperature of ground?
4)Why we cannot absorb carbon dioxide?Plant cannot absorb oxygen?
5)Why birds can fly in the sky?

Who can tell me?Thank you! 8)

2.

3. 1. Fire is hot simply because people chose to define hot based on fire.
2. Ice is cold because people chose to define cold based on ice.
3. Mountain temperature is lower because the sun's rays warm the air by warming the ground first. The further away from the "ground" you get, the colder the air is.
4. We actually do absorb carbon dioxide. However, it is "bad" for us. I'd imagine plants and animals evolved in such a symbiotic form so as to give balance to the Earth. If all creatures, both plants and animals, breathed the same things, it seems logical to conclude that eventually life would cease due to lack of material to breathe. Either that, or, if evolution is correct, all life would slowly convert to breathing the previous "waste" gases, and when those ran out, convert to breathing the most recent "waste" gases.
5. This is a very complex subject. A quick overview is that while flapping their wings, they create a pressure differential above/below the wings, where below is a higher pressure, and above is a lower pressure. In this way, they are able to "press" themselves higher into the air.

4. I no understand........ :?
Mountain close to the sun more than ground.Why sunlight warm the air by warming the ground first?

5. The air is less dense, so you have less molecules of air to contact you in the mountains.

6. The surface area of the ground at "ground level" versus the smaller surface area at 20000ft means there is more area to give off heat. On the mountain your only source of heat is the mountain, and whatever sunlight you can glean.

Here is a good link for you:
http://weatherquesting.com/cold-mountains.htm

7. I go to see the explain now.

8. I have go to see the explain.Now,I want to determine the reason.
Are the earth surface absorbs Sun's rays and re-radiates the heat that air can absorb,ground has more air than mountain?So,ground absorbs the sunlight's heat more than mountain? :wink:

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