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    A stone of mass 0.4 kg is projected horizontally at a speed of 6.0 m s−1 from the top of a wall, 5.0 m above the surrounding ground. When it arrives at the ground its speed is 10 m s−1. How much energy is lost by the stone in falling through the air?
    [1 mark]

    A 2.4 J

    B 6.8 J

    C 12.8 J

    D 14.4 J

    1/2mv^2 - 1/2mu^2 = Change in KE

    Hence, (1/2 x 0.4 x 10^2) - (1/2 x 0.4 x 6^2) = 20 - 7.2 = 12.8J

    You forgot to take into account the potential energy of the stone.
    Initial mechanical energy would be: (0.4)(9.8)(5) + (0.5)(0.4)(36) = 26.8 J;
    final mechanical energy would just be the stone's kinetic energy: 0.5(0.4)(100) = 20J

    hence delta m.e. is 6.8J
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