Question

why it is advised to tie any luggage on the roof of bus with a rope​

Answer 1

Answer:

It's because loading a bus with sum weight increases it's balance.

Answer 2

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.

While the bus is moving, luggage tends to remain in inertia of motion state  AND When the bus stops, the luggage tends to resist the change and due to inertia of motion it move forward and may fall off