Relative sizes of pymunk forces

Question

In Pymunk, is the magnitude of gravity the same as the magnitude of apply_force_at_local_point or apply_force_at_world_point, relatively speaking. In other words, is the magnitude of gravity=(20,40) equal to the magnitude of apply_force_at_world_point((20,40),object's position).

I used the equation of motion, final position = initial position + intial velocity * time + 1/2 * acceleration * t^2, to test that. It turns out that these magnitudes are not equal. For instance, it took a force of (0,-7888) to equal gravity of (0,-1750).

I am trying to determine apply_force_at_world_point force that would equal/cancel out gravity. I know I can just set the body's gravity to zero to achieve that effect but my goal is to determine magnetic force that would be enough to levitate a magnet of given weight and magnetic strength.

How can I find the magnitude of force (without testing a bunch of random values) that would equal gravity.

I hope the information given is enough to understand the issue

Answer 1

You can see how the velocity of a body is updated in the Chipmunk source code: https://github.com/viblo/Chipmunk2D/blob/master/src/cpBody.c#L501

body->v = cpvadd(cpvmult(body->v, damping), cpvmult(cpvadd(gravity, cpvmult(body->f, body->m_inv)), dt));

Translated to Python/Pymunk this would be something like this:

body.velocity = body.velocity * damping + (gravity + body.force / body.mass) * dt

From this I think this should work to make a opposing force matching the gravity:

body.apply_force_at_local_point(-space.gravity * body.mass)

(I tested this in a simple simulation with some gravity and a ball shape/body and it seems to work as expected, instead of falling the ball stayed put)