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Air Resistance

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Well I just recently was able to pull off jumping physics found here: http://www.gamedev.net/community/forums/topic.asp?topic_id=364490 And I thought air resistance would be easy to pull off, until I ran into this article: http://www.gamedev.net/reference/articles/article1615.asp And they said that air resistance is a complicated subject. How would I go about putting in realistic air resistance for falling objects?

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It depends on what exactly you are doing. For some things simple approximations give very good results, for other things you need a full blown fluid-dynamics solver to get realistic results. Or even a multiphysics solver if the falling thing changes it's shape due to forces inflicted by the air.

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try this

http://www.gamedev.net/community/forums/topic.asp?topic_id=71585
http://www.gamedev.net/community/forums/topic.asp?topic_id=55270

there was another thread about terminal velocities with wiki link,but i cannot find it atm.

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Yeesh, there's about a dozen different formulas, and I dunno which one to use. In my case, I'm doing it to a 2D object, so whatever formula that has anything to do with the area can be taking out of the picture. How would I go about using it on this simple formula?

v = v0 + a * t

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Quote:
Original post by Red_falcon
If my memory dont cheat on me;), is the normal air resistance based on area of object?


I've seen the actual formula before, but to be honest, I don't really remember it having to involve the area of the object. Although it seems you are on the right track, since different shaped objects are affected differently by air resistance.

If the object were to be 3D, I think it might have had something to do with the inertia, but I could be wrong. In this 2D case, I have no idea.

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In the rigid body simulator I'm developpin', I use the following formula to compute the amount of force the fluid is applying on my rigidbody:

F = 0.5*C*p*A*v²

C -> coefficient of friction of the surface (I usually use something like 0.3)
p -> fluid density(air density is aproximatelly 1.29kg/m³)
A -> surface area in m²
v -> surface velocity in m/s²

This way you get the force in Newtons that the fluid resistance is applying on the surface. Since in my rigidbody simulator I use convex polyhedra objects, I compute the force that is being applied on each triangle of the object and then I just sum all them. It gives me good results. I'm satisfacted with them.

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So earths air density is 1.29 kg/m3? Does that mean I have to make the constant 1.293 or just leave it at 1.29? Since I'm working in 2D, I'm not sure if I should make it squared instead of cubed.

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getting C out from the formula, you're assuming your object's surface has a coefficient of friction of 1.0. If you set C to 0.0, you'll get no friction meaning without friction, doesn't exist air resistance. You must put a value at there.

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Drag is a force that is related to several things:

  1. The size of the object

  2. The speed of the object (squared)

  3. The thickness of whatever the object is travelling through



If you treat the drag D as just another force (using the equations from our discussion on jumping), then the (simplified) equation for drag force D is this:

D = C * A * V^2;

Where A is the cross-section area of the object
V is the velocity of the object
C is a tuning constant, which should be whatever looks best.

Implement it with your other forces so that

F = D + G + O;

Where G is gravity, and O is other stuff.
Then get your acceleration as usual.

Is this related to your jumping objects?

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