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Kinetic and Static Friction

AP Physics 1· difficulty 2/5

m F f N W

A 4.0 kg4.0~\text{kg} block is pulled horizontally across a floor by a 20 N20~\text{N} force. The kinetic friction coefficient is μk=0.25\mu_k = 0.25. Using g≈10 m/s2g \approx 10~\text{m/s}^2, find the block's acceleration.

  • A

    3.5 m/s23.5~\text{m/s}^2

  • B

    7.5 m/s27.5~\text{m/s}^2

  • C

    5.0 m/s25.0~\text{m/s}^2

  • D

    2.5 m/s22.5~\text{m/s}^2

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Explanation

Normal force: N=mg=(4.0)(10)=40 NN = mg = (4.0)(10) = 40~\text{N}. Friction: fk=μkN=(0.25)(40)=10 Nf_k = \mu_k N = (0.25)(40) = 10~\text{N}. Net: Fnet=20−10=10 NF_{\text{net}} = 20 - 10 = 10~\text{N}, so a=10/4.0=2.5 m/s2a = 10/4.0 = 2.5~\text{m/s}^2.

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