Q: Why is it advised to tie any luggage kept on the roof of a bus with a rope ?
Sol: Luggage on the top of the bus is a loose fixture and not a compact part of the bus. Thus, when a speeding bus brakes suddenly, the luggage continues moving forward because of inertia of motion and is likely to fall off the bus. Conversely, if a stationary bus accelerates suddenly, the luggage continues in the same sate because of inertia, and hence is left behind relative to bus, such that it falls backward: To avoid the falling of luggage, it is tied with a rope.