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density of air at different temperatures

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From Yahoo Answers

Question:I am doing a science fair project on how the temperature of a tennis ball affects how high it will bounce. i need a decent answer with a lot of info so it can help me with my research part...........

Answers:as temperature increases, the atoms move faster in the tennis ball, increasing air pressure, expanding the ball since the ball is more expanded, the ball will have more bounce what you could do is buy identical tennis balls and record how high they bounce when dropped from a roof of something you would want to do tests where the temperature is at least 5-10 degrees different and drop test like 10 times with at least 5 different temperatures you want to drop it because the force in bouncing it will be different also, make sure that the ground has the same texture (if you tested dry, make sure it's dry in the other tests) you probably want to have someone help you record the height before you bounce you might want to let the balls sit for some time so it will have the same temp as the air

Question:I think I'm a little confused: How is it that warm air is less dense than cold air (i.e., temperature and density are inversely related), but both temperature and density decrease with increasing altitude (i.e., the two are directly related)? Can anyone help?

Answers:Its correct to say air density decreases with the increase of temprature but only in a constant pressure, but when we increase altitude, the main cause of a decrease in density is because of decrease in air pressure (PV=nRt) Or in other words, when P decreases, for a constant volume and gas, we have decrease in temperature You're not to mixed up, you just forgot to consider the air pressure factor! But that was a very good question!

Question:why does air (o2) become 'thinner' as elevation increases? It seems colder at higher elevations, so I would think that air condenses... -also, since heat rises, why it is colder on top of mountain than below it? (I took a trip to Mt. Everest and came back with these questions!:)

Answers:As we know, denser material is heaver. The force of gravity is amplified with mass. Also, the reason why it's colder is because there's less and less particles to absorb and trap heat that far up into the atmosphere, the environment is closer to that of a vacuum. Heat rises because heat disperses the molecules farther apart. Substances with lesser density will rise above substances with a high density.

Question:For a given temperature (let's say 25 C for instance) what is the difference in the air density: if relative humidity is 50%? if relative humidity is 100%?

Answers:If humidity increases,the density of air decreases. The density of water vapour is 5/8th of the density of dry air.So,the moist(humid) air which is a mixture of water vapour and dry air should have less density and therefore should be lighter than pure dry air.

From Youtube

Air density with temperature. Helium balloon. :With an helium balloon is easy to demonstrate that cold air is denser than hot air. With only 3 C of difference the ballon falls in warm air and lifts in cooler air.