Why is it advised to tie any luggage kept on the roof of a bus with a rope?
When a bus starts to move, it acquires a state of motion. But the luggage on the roof of the bus tends to stay at rest due to inertia.
Hence, when the bus moves, the luggage tends to remain at rest and hence falls back.
Also, when the bus stops, the luggage tends to remain in the state of motion due to its inertia and hence falls forward.
Therefore, in order to prevent the luggage from falling, it is advised to tie the luggage with a rope.