Why is it advised to tie luggage with a rope on the roof of buses?

State and explain the law of inertia (or Newton's first law of motion).

The inertia of an object tends to cause the object

Look at the diagram below and answer the following questions:
$(i)$ When a force is applied through the free end of the spring balance $A$,the reading on the spring balance $A$ is $20 \ g \ wt$. What will be the reading shown by the spring balance $B$?
$(ii)$ Write the reason for your answer.
$(b)$ If a balloon is filled with air and its mouth is untied,air is released from its mouth in the downward direction. Write the other observations made by you and justify your answer.

$(a)$ Define inertia. There are three solid balls, made of aluminium, steel, and wood, of the same shape and volume. Which of them would have the highest inertia? Why?
$(b)$ Describe in brief an activity to illustrate the property of inertia of rest.

The $SI$ unit of force is