It is advised to tie any luggage kept on the roof of a bus with a rope to prevent the luggage from falling off due to inertia when the bus suddenly starts or stops.

Explanation of the Principle

• When the bus starts moving, the luggage tends to remain at rest due to its inertia, so it may slide or fall backward off the roof.
• When the bus suddenly stops, the luggage tends to continue moving forward because of inertia, and can fall off the front of the roof.
• Tying the luggage with a rope provides an external force that secures it, counteracting the effects of inertia and keeping the luggage stable and in place during these changes in the bus's motion.

This principle is based on Newton's First Law of Motion, which states that an object resists changes to its state of motion unless an external force acts on it.