Determine the displacement $s$ in each of the following cases:
a $\frac{\mathrm{d}^{2} s}{\mathrm{~d} t^{2}}=-9.8$ given that at $t=0$, both $\frac{\mathrm{d} s}{\mathrm{~d} t}=0$ and $s=0$
b $\frac{\mathrm{d}^{2} s}{\mathrm{~d} t^{2}}=9.8 t$ given that at $t=0$, both $\frac{\mathrm{d} s}{\mathrm{~d} t}=0$ and $s=0$
c $\frac{\mathrm{d}^{2} s}{\mathrm{~d} t^{2}}=a(a$ is acceleration) given that at $t=0$, both $\frac{\mathrm{d} s}{\mathrm{~d} t}=u$ and $s=0$