How many pixels is an impulse in box2d

I have a ball that you blow on with air. I want the ball to be blown more if it is close to the blower and blown less if it is farther away from the blower. I am using box2d and I am us开发者_如何学Going the impulse function."body->ApplyLinearImpulse(force, body->GetPosition())". I can't seem to find a formula or a way to accomplish this. If I want the ball to blow to a total distance of 300 pixels right, how could I accomplish this? Please help.

If you want to calculate the distance before simulation you have to take a look at box2d sources. When simulating the velocity of the body is modified according to gravity, extra applied forces, linear damping, angular damping and possibly something more. Also velocity relies on velocity iterations.

But I think if you want a really smooth motion (like from a blow) you'd better use applyForce function instead of impulse. But be sure you are applying the force each simulation step.

EDIT:

Also you can simulate the air resistance as: Fa = -k*V*V. I've simulated movement in the pipe this way. Worked great.

So each step you can make something like this:

BlowForce = k1 / distance; // k1 - coefficient
Resistance = -k2 * V * V; //k2 - another coefficient
TotalForce = BlowForce + Resistance;
body->ApplyForce(TotalForce);

I am not a box 2d expert but what i would do is create a small box which is actually invisible and let the ball hit the box...if the blower is blowing more i would give more speed to the box in opposite direction. As far as 300 pixel length is concerned you have to adjust the forces and velocity such that the ball goes

300/<your_rendering_window_to_physics_world_ratio>

in physical world.

Force = mass * acceleration, so take the mass you set your body to, calculate the acceleration you want (remember to divide 300px by PTM_RATIO) and then multiply the two together.