A luggage is usually tied with a rope on the roof of buses. When a moving bus suddenly stops, the luggage on its roof tends to continue in the state of motion due to inertia of motion. Hence the luggage falls down from the roof of the bus. Similarly, when a bus suddenly starts, the luggage on the roof of the bus tends to continue in the state of rest and hence falls down from the roof of the bus. Thus, to avoid the falling of the luggage, it is tied with a rope on the roof of a bus.