The Momentum Pendulum

The momentum pendulum is shown in Figure 1.  A frame (red) to hold the pendulum was printed on a 3D printer.  The STL file in included with this lesson.  The frame is solidly attached to the PocketLab HotRod with three damage-free hanging strips.  A roughly 3" diameter  wood ball with a screw eye attached to the top of the ball is hung from a bifilar suspension so that the ball will swing in a plane.  Two small holes at the top of the frame provide an easy way to prepare the string suspension.  The smaller set of wheels are used with the HotRod, and PocketLab is attached to the HotRod as shown in the figure.  (The wood ball was purchased at a Michaels hobby store.)

The Momentum Pendulum Challenge

A suggested procedure for this challenge is as follow:

1. After explaining the construction of the momentum pendulum, ask the students to predict what will happen to the cart, if anything, when the pendulum is released from rest at an angle from the vertical.
2. Next, without using Voyager, let the students observe what actually happens.  Were their predictions correct?  Where there any surprising results that they did not predict?
3. Now ask the students to suggest what quantitative data could be captured by Voyager and the PocketLab app to provide a graphical description of the motion of the Hot Rod.  Have them predict what the graphs will look like.
4. Next, have the students do the experiment with Voyager mounted on the Hot Rod, collecting their suggested data with the PocketLab app.  Were their predictions on the shape of the graphs correct?
5. At what points in the swing of the pendulum does the Hot Rod suddenly change direction?  What is happening to the pendulum's momentum at these points?  The students should now be able to explain why this is called a momentum pendulum.

Momentum Pendulum Video

The 12-second video below shows the momentum pendulum in action.  The author of this lesson found it useful to collect data from PocketLab Voyager's rangefinder and accelerometer.   The white cardboard in the bottom left of the video is the zero reference position for the rangefinder.  The superimposed red line graphs the position of the Hot Rod, while the green line graphs the acceleration of the Hot Rod.  The grid in the background simply serves as a visual reference for viewing the motion of the Hot Rod.