Allowing objects to accelerate differentl using gravity

[eskimo456]

Hi there
I am Currently trying to work how how to allow gravity to affect different objects. Currently all objects fall at the same rate irregardless of mass ( as though in a vacuum). I want to be able to allow the objects to fall at different rates (as though on earth). I have tried changing the inertia and masses of the shape with no effect.

How would I for instance be able to simulate the different between a bowling ball being dropped and feather?

I understand that the air is a fluid and so a force is being applied to that effect the feather and bowling ball differently. Is there an easy way to approximate this in the bullet engine?

Currently I am trying

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Shape 1 shape->calculateLocalInertia(1,btVector3(0.5f,0.5f,0.5f));

and

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Shape 2 shape->calculateLocalInertia(1000,btVector3(0.5f,0.5f,0.5f));

I have also tried setting the inertia to 100 and the objects still seem to accelerate and fall equally. Do I need to apply an additional upwards force to simulate objects falling at different rates?

Many thanks

[mi076]

Is there an easy way to approximate this in the bullet engine?

the easiest way is to set linear/angular damping...
To get more accurate results write some approximation of fluid resistance..

http://en.wikipedia.org/wiki/Drag_equation

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    btScalar Cdrag = 0.5f * m_drag_coef * m_area * m_air_dens;
    btVector3 linear_velocity = body->getLinearVelocity();
    btVector3 Fdrag(0.0f,0.0f,0.0f);
    if (!linear_velocity.fuzzyZero())
    {
        btScalar drag = Cdrag*linear_velocity.length2();
        Fdrag = -1.0f*drag*linear_velocity.normalized();
    }