In the upper point, the velocity equals
"v_{up}=v_x=v\\text{ cos}\\alpha." The kinetic energy is
"KE=\\frac12 mv_x^2=\\frac12 mv^2\\text{ cos}^2\\alpha." The potential energy is
"PE=mgh," where the maximum height is
"h=\\frac{v^2\\text{sin}^2\\alpha}{2g},\\\\\\space\\\\\nPE=\\frac{mv^2\\text{sin}^2\\alpha}{2}." The ration of the energies is 0.2:
"0.2=\\frac{PE}{KE}=\\frac{\\text{sin}^2\\alpha}{\\text{cos}^2\\alpha}=\\text{tan}^2\\alpha,\\\\\\space\\\\\n\\alpha=\\text{atan}\\sqrt{0.2}=24\u00b0."
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