Boyant Force Questioni!^^ Again?? I already know that there were many posts relating to boyant force. The reason I know is because I read them all but still very confused so I need help from you guyz. T_T I am sorry for my bad English. My problem is with the explanation that is most commonly given out for the question why hot air rise. For me it is hard to apply the concept of boyant force to the system which is the collection of particles with higher kinetic energy. Why? Imagine there is a container, half filled by hot air molecules. Now to work out this problem using newton's law with boyant force concept, I need to have well defined system. To define system as the collection of particles with higher kinetic energy, I realize that I can't really define the volume of my system cuz the particles will swirl around in the certain region of box, will diffuse, and as guessed, will rise up. With out definite volume I can not give any means to its density. So I am stuck. I begin to doubt the concept of boyant force and density difference is not really explaning the question without skipping large step. Can somebody help me to understand? For all that is said, I have two specific question. Imagine we have a cubic container composed of gas. Let's say that there are many many gas molecules with low temperature. If we place the same gas molecule(Only one of it!) but with greater kinetic energy and release it from the very bottom of the container, will that one gas molecule rise up? My another question is this.... Imagine there is a cubic box. Box is half filled with gas with higher kinetic energy and half filled with lower kinetic energy. Initially hotter gas molecules and cold gas molecules are positioned through out the space proportionally. What I mean is....hmmmmm.....the density of number of molecules per volume of space is constant throught the region that is bounded by the cubic box. Now, do you think the gas molecules with higher kinetic energy will begin to rise???? (If so, isn'it violating the law's of thermodynamics?? decreasing the entropy???) Excuse me if I am not so clear in terms and definitions.