Luggage on the top of the bus is not a compact part of the bus. When a moving bus takes a sharp turn or suddenly applies brakes to stop, the luggage placed loosely on the roof of the bus continues moving forward (inertia of motion) and is likely to fall off the bus. Conversely, if a stationary bus accelerates suddenly, the luggage tends to remain in the same state (inertia of rest) and hence slides backward To avoid falling of luggage off the bus, it is advised to tie it with a rope.