9000117701

A body is thrown at the initial angle $\alpha ={30}^{\circ }$ and the initial velocity $v_0=20\mathrm{m}/\mathrm{s}$. The trajectory of the body is a part of parabola. Find the directrix of this parabola. Hint: The coordinates of the moving body as functions of time are $$\begin{array}{rlr}x& =v_{0}t\cdot \cos \alpha ,& \\ y& =v_{0}t\cdot \sin \alpha -\frac{1}{2}g{t}^2.\end{array}$$ Consider the standard acceleration due to gravity $g=10\mathrm{m}/{\mathrm{s}}^2$.