Maybe you have an older version with different behavior. To double-check just write something like
Code:

---

phent_settype(my, PH_RIGID, PH_SPHERE);
phent_addcentralforce(my,...);
wait(50);
phent_settype(my, 0,0);
wait(50);
phent_settype(my, PH_RIGID, PH_SPHERE);

---

And see if it still moves.