1. Description of Balanced and Unbalanced Forces
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Notice when the diver jumps down on a diving board. The board springs
back and forces the diver into the air. The action force exerted on the
board by the diver causes a reaction force by the board equal and opposite
to the force exerted by the diving board.
These interacting forces are unbalanced and differ from the balanced
forces you studied in Newton's first law. Recall that no motion occurs
when the forces are balanced. If the diver stands quietly on the diving
board, all the forces on the diver and the board are balanced. Neither the
diver not the board moves.