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

It is advised to tie any luggage kept on the roof of a bus with a rope to prevent it from falling off due to the principle of inertia. When the bus starts moving, the luggage tends to stay at rest because of inertia and may slide or fall backward. Similarly, when the bus suddenly stops, the luggage tends to continue moving forward and may fall off the roof. Tying the luggage with a rope provides the external force needed to keep it securely in place during sudden starts and stops of the bus.