While the bus is moving, luggage tends to remain in inertia of motion state. When the bus stops, the luggage tends to resist the change and due to inertia of motion it moves forward and may fall off. That’s why it is advised to tie any luggage kept on the roof of a bus with a rope. We also know, when the bus moves in forward direction, the luggage kept on the roof also moves along with the bus in forward direction. Now when the bus suddenly stops, it comes to rest but the luggage is still in forward direction due to inertia of motion, thus the luggage falls off the roof of the bus.So, it is advised to tie it.