box2d air resistance

box2d_air_resistance.java

float dragForce;
Vector2 appliedDrag = new Vector2();
float dragAngle;
float p, A, Cd, v; // elements of the formula see wikipedia entry for Drag (physics)
/*
ρ is the density of the fluid,
v is the speed of the object relative to the fluid,
A is the cross-sectional area
Cd is the drag coefficient – a dimensionless number.
sample drag co-efficients (http://en.wikipedia.org/wiki/Drag_coefficient)
    cube = 1.05
    sphere = 0.47
    streamlined-body (smooth wing shape) = 0.04
*/
// it's not necessary to calculate fluid density as I have, try 1.0 and change it until it looks nice
p = FLUID_DENSITY_OF_AIR * getAtmosphericDensity(ent.getAltitude()); // density of the fluid
// we can just leave this, cross-sections are beyond the scope of what we want to simulate
A = 1.0f;
// a very sleek object
Cd = 0.05f;
v = (float) Math.pow(ent.getSpeed(), 2); // speed squared
dragForce = 0.5f * p * v * Cd * A;
// we need the angle of the body's current velocity to know which angle we should set the drag force
dragAngle = ent.body.getLinearVelocity().angle();
// create a vector based on our dragForce
appliedDrag.set(dragForce, 0);
// align the drag to the same angle as our velocity
appliedDrag.setAngle(dragAngle);
// drag should slow down velocity when added to current velocity, so we make it negative
appliedDrag.scl(-1);
ent.body.applyForceToCenter(appliedDrag, true);