Naked Science Forum
General Science => General Science => Topic started by: Storm Steve on 06/10/2010 06:47:26
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[Copied and edited from an unanswered FaceBook post.]
My 13-year-old son asked me the following question:
Cold things fall and hot things rise. If I drop 2 identical objects, 1 cold and 1 hot, will they fall at the same rate or will the cold one hit the floor first?
I think his question is about buoyancy, which is a gravitational effect. If the density of the object is lower than the density of the surrounding atmosphere it will rise. The larger the difference in density the stronger the "upward force", opposing gravity.
Let's say that I have 2 glass marbles, one heated and one chilled. The hot marble will be slightly less dense than the cold one, assuming that glass expands when heated.
That means the hot marble should fall to earth slower than the cold one.
Or not?
Maybe the larger marble will have additional friction against the air?
What about other materials, other than glass?
Thanks for your insight!
Steve
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Hi Steve,
I think you have the right idea with the buoyant force and the larger marble falling slower.
Air resistance should be proportional to (very roughly) the cross-sectional area (width) of the object as it falls through the air. A larger object would therefore experience more air resistance and slow down more. This, together with the buoyant force, would probably make the hot marble fall very slightly slower.
These are tiny effects, though, so you probably wouldn't be able to notice a difference. The effect should be much clearer with a balloon, since gases tend to expand much more than solids. You could probably take two balloons which only had a little air in them. If you heat one up, it should noticeably expand (maybe try placing it in hot water?) Then you should definitely see the hot balloon fall more slowly than the cold one.
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What about drag ?
If the spheres were ball bearings, one heated red hot (+500oC), there would be transfer of heat from the hot ball to the air it was travelling through. The heated air would expand and have lower density which would reduce drag (http://en.wikipedia.org/wiki/Drag_coefficient#Definition), i.e. hot ball falls faster than room-temperature ball because of reduced drag, (until air flow it cools it).
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I agree with RD.
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What about drag ?
If the spheres were ball bearings, one heated red hot (+500oC), there would be transfer of heat from the hot ball to the air it was travelling through. The heated air would expand and have lower density which would reduce drag (http://en.wikipedia.org/wiki/Drag_coefficient#Definition), i.e. hot ball falls faster than room-temperature ball because of reduced drag, (until air flow it cools it).
Wouldn't the material of the (+500 deg C) ball expand from the heat, increasing the area of drag?
As it transfers the heat to the air around it, that air needs to displace the cooler air around it. Taking into account, these other factors, wouldn't this negate the drag theory and both fall at the same rate of speed
JP's idea sounds more practical, the medium density, (surrounding environment), is closer to the objects falling, difference of heat expansion would make a difference.
Try hot and cold water in two respective balloons place them in a pool of luke warm water.
Have we dismissed the effects of terminal velocity and when they come into play?
By definition... http://en.wikipedia.org/wiki/Buoyancy
"In physics, buoyancy (pronounced /ˈbɔɪ.ənsi/) is an upward acting force, caused by fluid pressure, that opposes an object's weight. If the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat. This can occur only in a reference frame which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction (that is, a non-inertial reference frame). In a situation of fluid statics, the net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body [1] This is the force that enables the object to float"
You can heat a still object, (object at rest), all day long and it may never float, and if so it can not be associated to have buoyancy.
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Wouldn't the material of the (+500 deg C) ball expand from the heat, increasing the area of drag?
As it transfers the heat to the air around it, that air needs to displace the cooler air around it. Taking into account, these other factors, wouldn't this negate the drag theory and both fall at the same rate of speed
Without making any computations? Are you a genius? [:)]
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Wouldn't the material of the (+500 deg C) ball expand from the heat, increasing the area of drag?
As it transfers the heat to the air around it, that air needs to displace the cooler air around it. Taking into account, these other factors, wouldn't this negate the drag theory and both fall at the same rate of speed
Without making any computations? Are you a genius? [:)]
Just asking
Is that being reasonable?
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Just asking
Is that being reasonable?
Don't know the exact meaning of "reasonable" not being very aknowledged with english, I can only say that it's "possible" but it depends on the object. For an iron ball (solid) I would bet on the fact the increased sectional area effect is less than the reduced density of the air, that is, the hot one would fall faster. With a hollow solid ball, it's another story, the increased dimensions for thermal expansion could be the dominant factor, but it depends on the total density of the ball: if the walls are very thin, you can be sure that the hot one would fall slower.
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Just asking
Is that being reasonable?
Don't know the exact meaning of "reasonable" not being very aknowledged with english, I can only say that it's "possible" but it depends on the object. For an iron ball (solid) I would bet on the fact the increased sectional area effect is less than the reduced density of the air, that is, the hot one would fall faster. With a hollow solid ball, it's another story, the increased dimensions for thermal expansion could be the dominant factor, but it depends on the total density of the ball: if the walls are very thin, you can be sure that the hot one would fall slower.
oK I forgot sorry disregard reasonable . As long as you understand I asked a question. No Genius identfication associated.
If you can calculate and maintain the heat in a hot object as it falls you can possibly answer the question.