Demos: 1N-06 Equality of Momentum (Cylinders)

Two wooden cylinders (one having a strong spring attached) of equal mass are suspended from the ceiling by bifilar suspensions. They are held together by compressing the spring and engaging a catch. The catch is then released by tapping on the metal “trigger,” causing the two cylinders to fly apart with equal speeds (which can be noted by the fact that each travels the same distance before returning. One of the cylinders is then replaced with one made of metal (it is actually shorter but much more massive). When the cylinders fly apart, the speeds are inversely related to the masses. The spring force that drives them apart is internal to the system, so the net momentum remains zero.

Directions: The two wooden cylinders will be hung and ready to engage. Insert the spike into the hole of the second cylinder and push the two firmly together until they lock. Carefully place the “knife blade” over the metal trigger and tap it quickly but firmly.

When replacing the wooden cylinder with the metal one, remove one wire at a time and place it on the metal cylinder. This avoids the certain entanglement that will occur if you remove all four of the wires before attaching them to the metal cylinder.

Suggestions for Presentation: Point out that the cylinders are the same mass and will be exploded apart by a spring. What do they expect will happen? (Usually they guess right.) Then replace the wooden cylinder with “a smaller one.” They might be surprised to see the “smaller one” move very little compared with the larger one.

Applications: If the metal cylinder were very massive (say, a 1000 to 1) compared with the wooden one, the metal cylinder would hardly move in the interaction. Compare this with the recoil of a rifle when a much lighter bullet is fired.

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