Answer
$y=- 5 \ \sin( \pi t)$
Work Step by Step
The object starts at the vertical position $y=0$, and moves in the downward direction.
The frequency is given by: $\omega=\dfrac{2\pi}{T}$
So, the function has the form (simple harmonic motion):
$y=-a \ \sin(\omega t)=-a \ \sin(\dfrac{2\pi}{T} t)$.
Here,
$y=-5 \times \sin (\dfrac{2\pi}{2} \times t)=- 5 \ \sin( \pi t)$
