Which of the following has the largest inertia?

Explain the following:
$(i)$ Why do the doors of a corridor train open or shut when the train starts or stops?
$(ii)$ Why is it necessary to run along with a moving bus in the same direction while alighting from it?
$(iii)$ Why does a player run for some distance before taking a long jump?
$(iv)$ Why does a person fall off the back of a stationary horse if the horse starts off suddenly?
$(v)$ Why is a neat hole formed in a glass pane if a bullet is fired at it from a close range?

$A$ body projected horizontally moves with the same horizontal velocity although it is under the action of the force of gravity,why?

$A$ block is supported by a cord $A$ from a rigid support,and another cord $B$ is attached to the bottom of the block. If you give a sudden jerk to $B$,it will break. But if you pull on $B$ steadily,$A$ will break. Why?

When a fast-moving horse stops suddenly,a careless rider falls in the forward direction. Give the reason for this.

An object experiences a net zero external unbalanced force. Is it possible for the object to be moving with a non-zero velocity? If yes,state the conditions that must be placed on the magnitude and direction of the velocity. If no,provide a reason.