you need to keep the wait(-1);// i gues you want to change it random every sec

you need somewhere else a loop

while(bpx == NULL){wait(1);)
while(box != NULL)
{
phent_addcentralforce (box, box_force);
wait(1);
}


(and you need to be sure that your box_force x and y are 0 else he could trip and move far far away)