Equation in the real world Does a quadratic equation like $x^2 - ax + y = 0$ describe anything in the real world? (I want to know, if there is something in the same way that $x^2$ is describing a square.)

Though not exactly same, depending upon value of a, following situations count as relevant.
For deep explanation, see wikipedia.


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*Bernoulli's Effect. This gives relation of velocity of fluid($u$), Pressure($P$), gravitational constant($g$) and height($h$),  $$\frac{u^2}{g}+P=h$$

*Mandelbrot Set has Recursive Equation $$P_c=z^2+c, z\in\mathbb{R}$$ which is interesting and creates fractals which appear in nature.

*The descrete logistics equation is a quadratic equation which surprisingly generates chaos. This is the way population growth (be it bacteria or humans) is calculated.
$$x_{n+1}=\mu x_n(1-x_n)$$ 

*Schrodinger's Equation

*Motion of Projectile as already mentioned
ed infinitum
