$A$ man weighing $50 \, kg$ lifts a weight of $30 \, kg$ to the top of a building $10 \, m$ high. Find the work done by him. (Take $g = 9.8 \, m s^{-2}$) (in $, J$)

List two conditions which need to be satisfied for the work to be done on an object.

The weight of a person on a planet $A$ is about half that on the earth. He can jump up to $0.4 \, m$ height on the surface of the earth. How high can he jump on the planet $A$ (in $, m$)?

An object of mass $10 \ kg$ is made to fall freely from a height of $10 \ m$. Complete the table:

Height of object $(m)$	$PE$ $(J)$	$KE$ $(J)$	Mechanical Energy $(J)$
$10$	...	...	...
$8$	...	...	...
$5$	...	...	...
Just above ground	...	...	...

Hence,state the law of conservation of energy.

The velocity of a body moving in a straight line is increased by applying a constant force $F$ for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.

$A$ boy of mass $40 \, kg$ runs up a flight of $50$ steps,each $10 \, cm$ high,in $5 \, s$. Find: $(i)$ the work done by the boy,and $(ii)$ the power developed. $(g = 9.8 \, m s^{-2})$