It is advised to tie luggage on the roof of a bus with a rope because of the law of inertia. According to Newton’s first law, an object at rest tends to remain at rest, and an object in motion tends to continue in motion unless acted upon by an external force.
If the luggage is not tied, it may slide or fall off when the bus starts, stops, or takes a turn suddenly. Tying it with a rope provides the external force needed to keep it in place.
Conclusion: Tying luggage prevents accidents and loss due to inertia.