C. As shown in Fig. 7, a 2 m long, 30 kg slender bar is constrained to move in the illustrated guide. Point B is able to mo ove in the vertical direction n, and A moves in the horizontal direction. At the instant shown, the spring is uncompressed, and the speed of B is 2 mis downward. The vertical guide provides a constraint force of 100N for B, and the spring constant is k = 100 N/m.
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