$(a)$ 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.

$(B)$ Inertia is the inherent property of a body to resist any change in its state of rest or uniform motion unless acted upon by an external unbalanced force.
Since all three balls have the same shape and volume, their mass depends on their density $(\text{Mass} = \text{Density} \times \text{Volume})$.
Steel has the highest density among aluminium, steel, and wood. Therefore, the steel ball has the highest mass and consequently the highest inertia.
$(b)$ Activity to illustrate inertia of rest:
$1$. Place an empty glass tumbler on a table and cover its mouth with a stiff playing card.
$2$. Place a five-rupee coin on the center of the card.
$3$. Give the card a sharp horizontal flick with your finger.
$4$. Observation: The card moves away quickly, while the coin falls vertically into the glass tumbler.
$5$. Conclusion: The coin remains in its state of rest due to inertia of rest, even when the card is removed suddenly.