At time t = 0 a particle starts travelling from a height 7 ^ z cm...

The position vector of the particle can be written as,

\overset{\to}{r} = 3t\overset{^}{i} + 5{t}^{3}\overset{^}{j} + 7\overset{^}{k}

The velocity of the particle will be,

\overset{\to}{v} = \dfrac{d \overset{\to}{r}}{d t} = 3\overset{^}{i} + 15{t}^{2}\overset{^}{j}

Now the acceleration of the particle will be,

\overset{\to}{a} = \dfrac{d \overset{\to}{v}}{d t} = \dfrac{{d}^{2}\overset{\to}{r}}{d{t}^{2}} = 30t\overset{^}{j}

At t = 1s ⇒ \dfrac{{d}^{2}r}{d{t}^{2}} = 30\overset{^}{j}