Demos: 1N-05 Elastic Collision (Magnets)

A pair of cylindrical permanent magnets are hung by bifilar suspensions from a framework in such a way that, at rest, they are horizontal and their axes are collinear. Two like poles face each other with a about a centimeter separation. If one magnet is held temporarily while the other is pulled back and released, an elastic “collision” is demonstrated, i.e. the first magnet stops and the second one takes off with the same speed the first one had prior to the collision. The demonstration shows that physical contact is not necessary for momentum to be conserved.

Directions: Check carefully for alignment before performing the demonstration; otherwise, wobbling will likely occur after the collisions. Hold one magnet while pulling the second one back a few centimeters. (Note: If you pull back too far, the magnets will actually come in contact with each other in a “normal “ collision. You might even want to do that first, to set the stage for the non-contact demo. And even though the like poles are facing each other, the second magnet tends to follow the first one when it is pulled back.) As you release the displaced magnet, let the other one go and you should get a nice elastic collision. But because of the interactions mentioned above, the system will start to behave erratically after about the second “impact.”

Suggestions for Presentation: Presumably you will have done the bowling ball collisions (1N - 2) and/or the Fun Balls demo (1N - 12). Ask what happens during an elastic collision and whether this might happen if the objects don’t touch. Then show the present demo. This sets the stage for asking the question, “What does it mean for objects to ‘touch’?” At the microscopic level, all contact involves electromagnetic interactions. Also, you might point out that momentum transfer and conservation requires only that there be a mutual interaction.

Applications: This is a good time to point out that “collisions” among atoms and molecules, etc., involve electromagnetic interactions and therefore momentum is conserved.

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