Question

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

Answer 1

When some luggage is put on the roof of a bus that is initially at rest, the acceleration of the bus in the forward direction will exert a force (in the backward direction) on the luggage. In a related way, when a bus that is initially in a state of motion quickly comes to rest due to the employment of brakes, a force (in the forward direction) is exerted on the luggage.

Depending on the mass of the luggage and the magnitude of the force, the luggage may fall off the bus due to inertia. Tying up the luggage will ensure its position and prevent it from falling off the bus.