Forces and Free-Body Diagrams

AP Physics 1· difficulty 3/5

A (4 kg) B (2 kg)

Block B (2 kg2~\text{kg}) rests on block A (4 kg4~\text{kg}), which sits on a frictionless floor. A horizontal force of 18 N18~\text{N} is applied to block A. Friction between A and B is sufficient that B does <strong>not</strong> slip. What horizontal friction force acts on block B from block A?

  • A

    12 N12~\text{N}

  • B

    0 N0~\text{N}

  • C

    6 N6~\text{N}

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  • D

    3 N3~\text{N}

Explanation

System acceleration: a=18/(2+4)=3 m/s2a = 18/(2+4) = 3~\text{m/s}^2. To accelerate B at 3 m/s23~\text{m/s}^2, friction on B must be f=mBa=(2)(3)=6 Nf = m_B a = (2)(3) = 6~\text{N}, directed forward.

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