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Rotational Equilibrium and Newton's First Law in Rotational Form

AP Physics 1· difficulty 3/5

A B W

A 4 m4~\text{m} uniform plank of weight 200 N200~\text{N} rests on two scales A (at left end) and B (at right end). A 100 N100~\text{N} box sits 3 m3~\text{m} from A. What does scale <strong>B</strong> read?

  • A

    100 N100~\text{N}

  • B

    175 N175~\text{N}

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  • C

    50 N50~\text{N}

  • D

    125 N125~\text{N}

Explanation

Torque about A: 4FB=200(2)+100(3)=700⇒FB=175 N4 F_B = 200(2) + 100(3) = 700 \Rightarrow F_B = 175~\text{N}.

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