See-Saw Center of Mass (1N20.10)
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The Reaction Carts are balanced on a long see-saw board so that their center of mass is directly above the fulcrum. The board is horizontal and stable. When the string between the carts is burned, they spring apart with equal and opposite momenta. The center of mass remains above the fulcrum so the board remains balanced (until one cart strikes the end of the board). Most dramatic with different mass carts.
Balancing the see-saw can be a little tricky so most instructors only do the demo once with unequal mass carts.