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Heat and weight->

  1. Dec 24, 2003 #1
    The hot air has less weight then the cold one.It's a fact but could it be that the hot air loses weight to gain heat?
  2. jcsd
  3. Dec 24, 2003 #2


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    It is not true that hot air has less "weight" than cold air. If you to take two equal samples of air and heat one, they would both still weigh the same.

    What happens is that the air that is heated will expand and become less dense. (The molecules making up the gas will increase their distance from each other.) This warm less dense air then tends to "float" on the top of the colder denser air.

    So heated air does not lose weight, it gains volume.
  4. Dec 25, 2003 #3
    I was thinking about Archimedes "Eureka" at the time of posting

    For a long time Archimedes was occupied with that problem. Then one day he tried to take a bath. While entering the jacuzy filed to the top with water he noticed that he is spilling water out. His idea was that his body and the supressed water take same volume. This would mean that their weights are proportional with their densities for the given equal volume. But what if those weights were different? He enters the water with 80kgs measured in air but he supress only 70kgs of water. What are those 10kgs of difference? And then he jumped yeiling "Eureka" = "I found it". What he did found we know from his famous quote:"If droped in water an object loses seemingly or actually as much weight as much is the weight of the supressed water".

    In a similar way I think while solving the main problem of this thread we should consider a const volume. The heat represents a weight of its own (you know the energy <=> mass and staff) and when it enters the given volume it supresses the air. Thus the remained air within the given volume due to its reduced quantity drops its weight and proportionally with it its density. So after all the give and take while gaining heat the air losses weight thus it elevates upwards.

    "heat and weight"
    is the name
    of the game
    now it's your turn
    to "heat and weight"
  5. Dec 25, 2003 #4
    First, I think you have to be a little more carefull of when to use effects of relativity in an experiment. Even though they are happening all the time and everywhere, their effects will be noticed more in experiments of very, very high speed.

    As an example, I ask a friend to go on his car to order pizza, but before he leaves, I ask him the time to set my clock to exact the same time he is measuring. After he comes back with the pizza, I could assure you that the time on his clock will be the same time on my clock. Even though relativity says that since he was moving, his time is different than mine, which is true, but the difference in time is so small, that we can neglet it.

    To my little knowledge, this whole energy <=> mass thing is an effect of relativity. Therefore, in experiments where we are not traveling near the speed of light, we can say that energy and mass are constant (can't be created or destroyed) and that there is no transfer from one to the other (well, maybe some transfer will occur, but it will be so few that we can neglet it).

    Second, you need to learn to differenciate between mass and weight of an object. This statement still puzzles me: "But what if those weights were different? He enters the water with 80kgs measured in air but he supress only 70kgs of water." Try to define both of them, and then find the units of measure of both (Kgs is a unit of mass).

    After all this, try to think of it again. Maybe then we can discuss the Archimedes history, experiments and principles.
  6. Dec 26, 2003 #5
    I wasnot refering to energy-mass conversion but their proportionality. So no SR effects can be found in my previous post.
    Again weight is an force proportional with mass. So in fact every phenomenon can be viewed only by cahrge, mass and distance. Those are the basic qualities of the matter.

    The difference of 10kg might show if the water and the humman body have different densities when ocupying same volume:
    I'm ready!!
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