Why is cold air "heavier" than hot and does this explain why black holes are heavier than hot stars ?

Question

If cold air sinks why not too black holes and is "coldness" itself gravity explained, in a nutshell ?

Answer 1

Cold air is not heavier, it is denser. In a warm air sample, the gas molecules are vibrating with a higher speed, meaning they require more space. The same number of gas molecules will thus occupy a larger volume, and hence have a lower density.



"Coldness" is a lack of thermal energy. It has nothing to do with gravity as such.



Black holes don't "sink" because in space, there is no up or down. There is no reference frame to check if something is sinking or not.

Answer 2

A volume, V, of gas, weighs W = rho g V; where rho is the mass density, g is g.



The density of cold air is Rho > rho for the same gas when it's hot. That results because hot air molecules have more kinetic energy and zing about randomly over longer rms tracks. That is, hot air molecules are not packed in so tight as the cold molecules because they need more room to move around in.



So a given volume V of cold air weighs W = Rho g V > rho g V = w and clearly W > w in this case. The cold air is heavier than the hot air.



The hot air floats atop the cold air because it is indeed lighter than a comparable volume of cold air. Conversely, the cold air sinks below the hot air because it is heavier.



I'm puzzled by the leap from air to black holes. The physics and natural laws governing the two are totally different. And I think you need to study up on BH because you have some misconceptions.



First, BHs are not black because they are cold. In fact, the temperatures inside a massive BH can approach the trillions of degrees K that were once commonplace during the big bang. But they are black, from the outside, because all that radiant energy is trapped by the immense gravity and none escapes to be seen.



Second, as someone else has pointed out, there is no up or down for the typical natural BH. They are in deep space, where up and down have no meaning as there is no common reference point to use.



Sorry, your attempt to attribute the force of a gravity field to the degree of coldness will receive an icy reception in the physics domain I'm afraid.

Answer 3

cold air is denser than hot air, Thus for the same volume cold air is heavier than hot air (v=mass*density). You can use the same argument to explain weight of black holes and stars.