Answer
$y=-5\cdot \sin(\pi{t})$.
Work Step by Step
The object starts at the vertical position $y=0$, and moves down.
By definition, the frequency is $\omega=\frac{2\pi}{T}$, hence the function is: $y=-a\cdot \sin(\omega t)=-a\cdot \sin\left(\frac{2\pi}{T}\cdot t\right)$.
Here, we have
$y=-a\cdot \sin\left(\frac{2\pi}{T}\cdot t\right)=-5\cdot \sin\left(\frac{2\pi}{2}\cdot t\right)=-5\cdot \sin(\pi{t})$.
