Physics - addThruster

[moneim230]

Hello,
i applied a constant force on an object and i wanted to change its value

F=[0,0,0]
def UpdateThruster():
	force=ball.addThruster(mode=viz.ABS_GLOBAL,force=F)
vizact.ontimer(0,UpdateThruster)

def onKeyDown(key):
	if key == ' ':
		F[2]=1

viz.callback(viz.KEYDOWN_EVENT,onKeyDown)

i found that the force is accumulated each time the function UpdateThruster happens
so, when i press space once, the ball starts accelerating very fast
please i need to apply a force and change its value anytime i want...how can i do this ??