Air friction and speed & temperature

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1. It makes sense that air drag increases with speed, but is it a direct increase or to the square of the velocity? (so the faster you go the more energy wasted?)

2. What is the relationship between air temperature, pressure, and air drag? I can see how on a warmer day the molecules are further apart but moving much quicker. Thus less air drag.

3. I asked my professor about some of this last night but he had a bit too many drinks because of our graduation, so he didn't make much sense.
However, he explained something interesting about how molecules repel each other (when they come in 'contact'). He explained that if you come closer to the molecule at a low speed, it will be repelled without you losing too much energy. But if you were moving very quick, you'd get much closer to the air molecules and they'd bounce off with a greater speed, hence you'd be experiencing a higher energy loss than you would at lower speeds (because of air drag)
Something similar to that I guess.
 

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  • #2
Simon Bridge
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1. It makes sense that air drag increases with speed, but is it a direct increase or to the square of the velocity? (so the faster you go the more energy wasted?)
Drag models can be quite complicated - but they usually boil down to a drag force being proportional to the square of the speed acting opposite the direction of the velocity.

2. What is the relationship between air temperature, pressure, and air drag? I can see how on a warmer day the molecules are further apart but moving much quicker. Thus less air drag.
Higher pressure means more drag - basically, drag comes from an object having to drag a volume of air behind it rather than from skin friction with air molecules.
NASA has some good pages on this.

3. I asked my professor about some of this last night but he had a bit too many drinks because of our graduation, so he didn't make much sense.
However, he explained something interesting about how molecules repel each other (when they come in 'contact'). He explained that if you come closer to the molecule at a low speed, it will be repelled without you losing too much energy. But if you were moving very quick, you'd get much closer to the air molecules and they'd bounce off with a greater speed, hence you'd be experiencing a higher energy loss than you would at lower speeds (because of air drag)
Something similar to that I guess.
Is there a question there?
 
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Is there a question there?

Yeah, sorry. I forgot to ask how much of that is correct and an elaboration.
Thanks for your response.
 
  • #4
boneh3ad
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Higher pressure means more drag - basically, drag comes from an object having to drag a volume of air behind it rather than from skin friction with air molecules.
NASA has some good pages on this.

It's both, actually, and the two can be related.
 
  • #5
Stephen Tashi
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Higher pressure means more drag - basically, drag comes from an object having to drag a volume of air behind it rather than from skin friction with air molecules.
NASA has some good pages on this.

Maybe there are some bad pages too. From the page http://www.space.com/15353-meteor-showers-facts-shooting-stars-skywatching-sdcmp.html
Meteor showers occur when dust or particles from asteroids or comets enter Earth's atmosphere at very high speed. When they hit the atmosphere, meteors rub against air particles and create friction, heating the meteors. The heat vaporizes most meteors, creating what we call shooting stars.

But we read elsewhere ( http://www.slate.com/blogs/bad_astr...l_meteor_leaves_a_bright_trail_behind_it.html ) that it isn't "skin friction" that is the main cause for heating meteoroids.

As a meteoroid (the actual solid chunk of material) blasts through the atmosphere, it violently compresses the air, heating it up hugely (note this isn’t due to friction, but compression; like when a bicycle pump heats up as you use it)

Edit: Also the NASA page: http://solarsystem.nasa.gov/planets/meteors/#! says:
Little chunks of rock and debris in space are called meteoroids. They become meteors -- or shooting stars -- when they fall through a planet's atmosphere; leaving a bright trail as they are heated to incandescence by the friction of the atmosphere.
 
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boneh3ad
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Like I said, it's a little of both, and exactly how much of each is based on the Mach number (primarily) and other factors such as viscosity and density and things like that. Typically most of the actual heading of the air is due to compression. There is some heating of the air due to "friction". I put that in quotes because the concept of friction doesn't truly make sense here. The actual mechanism is viscous dissipation. As the air is slowed down by viscosity in the vicinity of the object (relative to the object), some of that energy is dissipated as heat. How much exactly is rather complicated, but generally, it is going to be far less than heating due to compression at high Mach numbers.

However, that is all about how much the air surrounding the object heats up. If you want to talk about how fast the object itself heats up, that has everything to do with viscosity ("friction"). Heat transfer through a fluid into the object would occur even in still air via conduction, but air is a pretty poor conductor, as I am sure you are aware. However, add in air movement and viscosity and now we are talking about convection, and the heat transfer is greatly increased. If the air flow over the surface is turbulent (as opposed to laminar), then you are talking an additional tenfold increase in heat transfer.

So, in that sense, how hot a meteor gets (and how fast) has everything to do with friction. It also has everything to do with compression. The answer is both.
 
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