I read about this on another post, and it got me interested. suppose you have a pendulum (say a steal bar with a weight at the bottom or something) and a spring attached to the bottom. now you take the spring and pull it all the way down. you should have the pendulum pointing straight down, and the spring extended unerneath. what happens when you release the spring? note: it is not an 'ideal' pendulum, i.e. it is not pointing 'straight' down. therefore the spring doesnt just oscilate up and down, the pendulum moves a bit too i think.