The ends of a beam of length $L$ rest on two supports as shown in FIGURE 5.1 .6 . With a uniform load on the beam, its shape (or elastic curve) is determined from
$$
E I y^{\prime \prime}=\frac{1}{2} q L x-\frac{1}{2} q x^{2}
$$
where $E, I,$ and $q$ are constants. Find $y=f(x)$ if $f(0)=0$ and $f^{\prime}(L / 2)=0 .$