Question

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

Answer 1

When the bus starts the lower part of the luggage being in contact with the bus attains motion but the upper part od the luggage tends to be at rest. Due to inertia of rest it falls backward. To avoid the falling it is tied with a rope. I hope it helps u

Answer 2

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The luggage kept on the roof possess inertia. As the bus moves, brakes to stop or takes a true the luggage might fall off. Thus, it should be tied with a roof.