Thanks all for the replys.
CPB probably spotted that I am not a pilot. It was more of a theoretical question trying to figure out what forces the slip instrument measures. ( gravity and centripetal?)
Maybe this would have been a possible question to answer? If a twin has one engine out but is flying in a straight line with wings level a yaw string would show it was cocked off to one side, while the ball would be centred?
Thanks for your help.