Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Explanation

When the bus starts suddenly, the lower part of the luggage kept on the roof being in contact with the bus begins to move forward with the speed of bus, but the upper part tends to remain at rest due to inertia of rest. Therefore, the upper part is left behind and hence luggage falls backwardd. So, it is advised to tie any luggage kept on the roof of a bus with a rope.