If the initial velocity in horizontal direction of a projectile is unit vector $\hat{i}$ and the equation of trajectory is $y=5 x(1-x)$. The $y$ component vector of the initial velocity is ______________ $\hat{j}$. ($\mathrm{Take}$ $\left.\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^{2}\right)$