Q 3. Why is it advised to tie any luggage kept on the roof of a bus with a rope?
This is because the luggage has some mass so the inertia. When bus is in motion, the luggage is also in the motion. But when the bus stops suddenly, the luggage still tends to move with the same velocity due to inertia. Thus it may fell down. Similarly when the bus starts from the rest, the luggage tends to be in rest thus it is pushed in backward direction.