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.)
 A: The height $y(t)$ in meters of an object falling under gravity, after $t$ seconds, is given by
$$y(t) = -4.9t^2 + v_0 t + y_0,$$
where $y_0$ is the object's initial height and $v_0$ is the object's initial vertical velocity. In particular, solving for $y(t)=0$ tells you when the object will hit the ground.
A: Though not exactly same, depending upon value of a, following situations count as relevant.
For deep explanation, see wikipedia.


*

*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
A: Equation  $x^2 - ax + y = 0$ represent for example ($a$ is real parameter) a families of parabolas. Parabola is trajectory of canon ball. 
