Physics

Problem

A train is travelling on a straight track at

12\,\text{m/s}

. After

4\,\text{s}

its velocity becomes

20\,\text{m/s}

. It then slows down to

8\,\text{m/s}

in the next

5\,\text{s}

. Find the average acceleration of the train for the entire

9\,\text{s}

interval.