Why is it advised to tie any luggage kept on the roof of a bus with a rope?

(N/A) When the bus accelerates and moves forward,it acquires a state of motion. However,the luggage kept on the roof,owing to its inertia,tends to remain in its state of rest. Hence,with the forward movement of the bus,the luggage tends to remain at its original position and ultimately falls from the roof of the bus. To avoid this,it is advised to tie any luggage kept on the roof of a bus with a rope.