Conservation of Energy

AP Physics 1· difficulty 4/5

L

A pendulum bob of mass mm on a massless string of length LL is released from rest with the string horizontal. What is the tension in the string when the bob passes through the lowest point?

  • A

    T=12mgT=\tfrac{1}{2}mg

  • B

    T=3mgT=3mg

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  • C

    T=mgT=mg

  • D

    T=2mgT=2mg

Explanation

Energy: 12mv2=mgL\tfrac{1}{2}mv^2=mgL, so v2=2gLv^2=2gL. At the bottom, Tmg=mv2L=2mgT-mg=\dfrac{mv^2}{L}=2mg, giving T=3mgT=3mg.

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