From the movie, it should be obvious that the center of mass of the two pucks moves along a straight line. This was our objective and could stop here. However, because this movie has some anomolies, lets check out the plot of the momentum of the center of mass.

  • Click on the Graph icon in the toolbar
  • Leave the Horizontal Axis as Time
  • Set the Vertical Axis to "CM 1" "magnitude" "Momentum"
  • Click "OK" to create the graph

You will probably notice some peaks in this graph, which you would expect to be constant for an elastic collision. If you click on one of the points in the peak, it will advance the movie to that frame. Looking at the movie, you should see some gaps in the path of the center of mass on the movie. This indicates that either

  1. the puck got a boost of energy (maybe it sneezed?)

    2. or
  2. the time code on the movie could be incorrect.
Looking at the data table, the time code on the movie shows consistent times; this is obviously not the case.


note the two gaps in the data. One is by the selected point; the other is indicated by the cursor on the movie


This is something you will have to look out for when making your own movies; occasionally, if a frame is dropped, this type of error in the data can occur.

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