Question

why is the luggage kept on the roof of the bus tied with a rope

Answer 1

luggage kept on the roof of the bus is tied with a rope because at first, both the bus and the luggage is at rest and if suddenly the bus starts then the luggage will fall backwards because of inertia of rest and when both the luggage and the bus are in motion and suddenly the bus stops, then the luggage will fall because of inertia of motion.

Answer 2

There might not be enough space on the bus if the luggage is kept inside. So it is kept o the roof. It is tied on the roof so that it does not fall onto the road.