M-EngMom5: Conservation of energy; example: pendulum with changeable length.

PIRA: Unknown

Equipment: Ivory ball (heavy, white) attached to a strong fishline located in Set 10, Cabinet 3, Shelf 3.

Procedure: Mount the ivory ball as a pendulum attached just above the blackboard so that it swings in a plane parallel to the blackboard and very close to it. At some convenient height fairly high on the board draw a horizontal chalk line. Pull the ivory ball to one side and release it from a point on this horizontal line so that the ball swings freely just in front of the board. Call attention to the fact that the ball rises almost to the line on the far side of the swing. While the pendulum is still making essentially the full swing, take a short stand rod and push it firmly against the board at a point directly below the point of suspension. As the pendulum swings back the cord will strike the rod. The ball will continue its swing with reduced radius of curvature. Note, however, that the ball will rise to approximately the same height as indicated by the horizontal line.

If the position of the rod is sufficiently low the ball will loop the loop around the rod. (Remember that in order to loop the loop the ball must pass over the top with considerable speed. If there is insufficient speed the path will be a parabola making an inner tangent with the expected circle, thus putting much slack in the cord. As it falls along this new curve, there comes a time when this slack is taken up suddenly, often breaking the cord).