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17 July, 10:10

A rock is dropped from rest from a height h above the ground. It falls and hits the ground with a

speed of 11 m/s. From what height should the rock be dropped so that its speed on hitting the ground

is 22 m/s? Neglect air resistance

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Answers (1)
  1. 17 July, 11:51
    0
    Use energy conservation, since no energy is lost it must be constant.

    E = 0.5mv² + mgh

    At release the velocity v = 0 and the height is h.

    E = 0 + mgh

    At impact the height h = 0 and the velocity is v.

    E = 0.5mv² + 0

    Since the energy E is conserved:

    0.5mv² = mgh

    the mass m cancels and the equation becomes:

    0.5v² = gh

    h = 0.5v²/g

    when g = 9.81 and v = 22:

    h = 24,66
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