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

Explanation

Because it may slide and fall due to following reasons:
(i) Initially, if the bus is in the state of rest, the luggage is also in the same state of rest. When the bus starts suddenly the luggage tends to remain in the state of rest due to inertia of rest. As a result, the luggage can be thrown in the backward direction and it may fall.
(ii) If the bus is in the state of motion, the luggage is also in the same state of motion due to inertia of motion. So, when the brakes are applied it may fall in the forward direction.
(iii) If the bus takes a sharp turn on a road, the luggage will resist any change of its state of direction due to inertia. As a result the luggage can be thrown sideways and may fall. Therefore, it is advised to tie the luggage kept on the roof of the bus to prevent it form falling.