Rotational Equilibrium and Newton's First Law in Rotational Form

AP Physics 1· difficulty 4/5

A uniform plank of mass M=20 kgM = 20\text{ kg} and length L=4 mL = 4\text{ m} rests on two supports: one at the left end and one 1 m1\text{ m} from the right end. A person of mass 60 kg60\text{ kg} stands at the right end. What is the upward force from the right support? (Use g=10 m/s2g = 10\text{ m/s}^2.)

  • A

    500 N500\text{ N}

  • B

    667 N667\text{ N}

  • C

    800 N800\text{ N}

  • D

    933 N933\text{ N}

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Explanation

Sum torques about the left end: FR(3)=(200)(2)+(600)(4)F_R(3) = (200)(2) + (600)(4), so FR=2800/3933 NF_R = 2800/3 \approx 933\text{ N}.

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