# Air friction and speed & temperature

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.

Simon Bridge
Homework Helper
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?

Is there a question there?

Yeah, sorry. I forgot to ask how much of that is correct and an elaboration.

Gold Member
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.

Stephen Tashi
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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