[Pyode-user] Collision detection works fine in one direction, but not the other

[George <pin...@gm...>]

import ode

BD=.0615
BR=BD/2

def c_callback(args, geom1, geom2):
    contacts=ode.collide(geom1, geom2)
    world, contact_group = args
    for c in contacts:
        c.setBounce(.7)
        c.setMu(5000)
        j = ode.ContactJoint(world, contact_group, c)
        j.attach(geom1.getBody(), geom2.getBody())

world=ode.World()
space=ode.Space()
contact_group=ode.JointGroup()

world.setGravity((0,-9,0))

top_rail=ode.GeomPlane(space, (0,1,0), 1.42)
bottom_rail=ode.GeomPlane(space, (0,1,0), 0)

bballs=[ode.Body(world) for i in range(1)]
for bball in bballs:
    M=ode.Mass()
    M.setSphere(2500., BR)
    bball.setMass(M)
    geom=ode.GeomSphere(space, BR)
    geom.setBody(bball)
for bball in bballs:
    bball.setPosition((1,0.8,1) )
    bball.addForce((0,0,0))

dt=.01
while(1):

    space.collide((world,contact_group), c_callback)
    world.step(dt)
    print bballs[0].getPosition()
    contact_group.empty()


The above works fine. But if I change the ygravity from -9 to 9 the
ball doesn't bounce on the top_rail, it just goes through. Why?