1M40.15 Stopped Pendulum


Conservation of mechanical energy


A pendulum is released and the height of the swing is recorded. A rod is then placed in the way of the pendulum and the process is repeated. The height of the swing with the rod is the same as without the rod.



  • [1] 1.5" diameter wooden ball on a string
  • [1] Magnetic mount
  • [1] Rod on a magnetic mount
  • [1] Chalk
  • [1] Metre stick
  • [1] Magnetic blackboard or whiteboard

Important Notes

  • Practice this demo in advance to drop the ball smoothly and avoid hitting the blackboard
  • Do not use a heavy bob as it may pull free from the mount or the blackboard


  1. Mount the pendulum on the blackboard.
  2. Raise the pendulum and mark the initial height on the blackboard.
  3. Release the pendulum.
  4. Mark the height of the swing on the blackboard.
  5. Place the rod below the pendulum mount so that it is in the path of the pendulum.
  6. Raise the pendulum to the marked initial height and release. The pendulum will swing to the same height as before.

The pendulum may be made to loop the rod if the height of release and the height of the rod are chosen appropriately.


Additional Resources


  • PIRA 1M40.15
  • VideoEnc 03-13
  • Sutton M-132;DHP Mr-3; DaR M-414; JSMiller p102
  • From Galileo Galilei "Dialogues Concerning Two New Sciences" (Leyden, 1638) trans Henry Crew and Alfonso de Salvio (Dover 1954):
    "Imagine this page to represent a vertical wall, with a nail driven into it; and from the nail let there be suspended a lead bullet of one or two ounces by means of a fine vertical thread, AB, say from four to six feet long, on this wall draw a horizontal line, DC, at right angles to the vertical thread AB, which hangs about two finger-breadths in front of the wall. Now bring the thread AB with the attached ball into the position AC and set it free; first it will be observed to descend along the arc CBD, to pass the point B, and to travel alongthe arc BD, till it almost reaches the horizontal CD, a slight shortage being caused by the resistance of the air and the string; from this we may rightly infer that the ball in its descent through the arc CB acquired a momentum [impeto] on reaching B, which was just sufficient to carry it through a similar arc BD to the same height. Having repeated this experiment many times, let us now drive a nail into the wall close to the perpendicular AB, say at E or F, so that it projects out some five or six finger-breadths in order that the thread, again carrying the bullet through the arc CB, may strike upon the nail E when the bullet reaches B, and thus compel it to traverse the arc BG, described about E as center. From this we can see what can be done by the same momentum [impeto] which previously starting atthe same point B carried the same body through the arc BD to the horizontal CD. Now, gentlemen, you will observe with pleasure that the ball swings to the point G in the horizontal, and you would see the same thing happen if the obstacle were placed at some lower point, say at F, about which the ball would describe the arc BI, the rise of the ball always terminating exactly on the line CD. But when the nail is placed so low that the remainder of the thread below it will not reach to the height CD (which would happen if the nail were placed nearer B than to the intersection of AB with the horizontal CD) then the thread leaps over the nail and twists itself about it."


  • Don't attempt this at home!

Last revised

  • 2018


  • Original construction: the magnetic mounts were purchased. An eye hook was screwed into the wooden ball and a string was attached. The rod is a 4" length of aluminum bar.

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If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.