Book: # 3

This film comprises a set of nine two-body collisions between pucks on an air table.  Because the pucks are free to move in two dimensional space, these collisions provide us with an opportunity to study the vector nature of momentum conservation.  That momentum should be conserved in a collision is perhaps most easily argued if we consider the system as a whole.  In the absence of any external force, Newton's Law tells us that there is no change in momentum and, when we follow the path of the center of mass of the colliding pucks, we see that this is true. 

But, we know that the momentum of each individual part of the system can and does change.  If the momentum of the system as a whole doesn't change, however, the momentum changes of the parts of the system must cancel each other out.  The sum of the momenta of the two pucks after the collision must equal the momentum of the projectile puck before the collision.

This is a vector relationship, which means that if we resolve the momenta of the two pucks after the collision into components in line with and perpendicular to the path of the projectile puck, the in-line components should sum to the initial momentum and the perpendicular components should be equal and opposite.  In each of the collisions shown in the film, the two pucks leave a trail of "ghosts" at equal time intervals.  When the distance numbers (measured from the point of impact) and the angle measures appear, press the stop-motion button of your projector and answer the appropriate questions.  (See inside of film case for study questions.)