$A$ girl is carrying a school bag of $3 \, kg$ mass on her back and moves $200 \, m$ on a levelled road. The work done against the gravitational force will be $(g = 10 \, m \, s^{-2})$.

Compare the power at which each of the following is moving upwards against the force of gravity? (given $g = 10 \, m \, s^{-2}$)
$(i)$ $A$ butterfly of mass $1.0 \, g$ that flies upward at a rate of $0.5 \, m \, s^{-1}$.
$(ii)$ $A$ $250 \, g$ squirrel climbing up on a tree at a rate of $0.5 \, m \, s^{-1}$.

How is work done by a force measured? $A$ porter lifts a luggage of $20 \, kg$ from the ground and puts it on his head $1.7 \, m$ above the ground. Find the work done by the porter on the luggage. $(g = 10 \, m s^{-2})$

Define work. How is work measured? When is work done by a force negative?

State whether the following statement is true or false:
Work done by centripetal force is zero.

Two bodies of equal masses move with the uniform velocities $v$ and $3v$ respectively. Find the ratio of their kinetic energies.