Why does a force have no effect after a voppop?

In the attached file a voppop set the velocity of a particle (0, .1, 0).
A force is added (.2, 0, 0).
If the voppop is bypassed the particle is affected by the force.
If the voppop is active the force is not affecting the particle.

Why?

(this is a h8 file, as my h9 refuses to create new voppops. I submitted this as a bug)

Georg

The force POP changes acceleration which is used to update the velocity at the start of the next frame. Your VOP POP, on the other hand, is overwriting that velocity with (0, 0.1, 0) every frame.

ok. I understand.