Swings, the calming effect of going back and forth. The swing is a very common thing to see at a playground of on a front porch. It is basically a pendulum. But it could also be a mass on a tension. The chain or rope has to hold the swing, but also the mass that sits or stands upon it. This simple device used to please kids have ruined may high schoolers lives by showing up on tests and ruining their grade, the the circle has in math. Now very nice in concept of using the math that has to be done with it can be brutal. The acceleration and the force which it needs to be pushed to keep it at a cons ten speed all can be calculated with certain equations.