$(a)$ Define power. Give its $SI$ unit.
$(b)$ Taking the example of a simple pendulum, explain the variations in the forms of energy and the interconversions involved.

(N/A) Power is defined as the rate of doing work. Its $SI$ unit is watt $(W)$.
$(b)$ For a simple pendulum, the interconversion of energy is as shown in the table below:

Position	Potential Energy $(PE)$	Kinetic Energy $(KE)$
Extreme position $1$	Maximum	Zero
Between extreme $1$ and mean	Decreasing	Increasing
Mean position	Zero	Maximum
Between mean and extreme $2$	Increasing	Decreasing
Extreme position $2$	Maximum	Zero

As the pendulum swings, the total mechanical energy $(PE + KE)$ remains constant, demonstrating the law of conservation of energy.