Question

why it is addvised to tie any luggage kept on roof of a bus with rope
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Answer 1

a luggage has to be tied with a rope in a roof of the bus because when a bus turns along the steep road, it changes its direction,this is according to inertia of direction. when a bus turns, the luggage may change its direction and will fall..

Answer 2

When the bus stops suddenly, the luggage on the roof top will fall forward due to inertia of motion. Similarly when the bus starts the luggage will fall backwards due to inertia of rest. To avoid this, any luggage kept on the roof of a bus is tied with a rope.