PURPOSE: To show that the center of mass may remain at rest during motion within the system.

DESCRIPTION: The board is balanced on its fulcrum with the carts touching at the center of the board. Releasing the spring causes the carts to push against each other and separate, but the board remains balanced.

SUGGESTIONS: The experiment can be made more complex by putting an extra weight into one cart, so that the masses of the carts are unequal.

For hints on using this demonstration to enhance classroom involvement, see Question of the Week #188.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Two PSSC carts, balance board with fulcrum, and stick to release cart spring; extra weight available on request.

SETUP TIME: 5 min.