$A$ 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 $, J$)

$(a)$ What is the commercial unit of energy? Give its relationship with the $SI$ unit of energy.
$(b)$ An electric bulb of $60 \,W$ is used for $10$ hours every day. How many units of electrical energy are consumed by this bulb in one day?

$A$ boy throws a rubber ball vertically upwards. What type of work is done: $(a)$ by the force applied by the boy,and $(b)$ by the gravitational force? Support your answer with reasons.

$(a)$ Give two conditions for work to be done.
$(b)$ What is the work done by frictional force on an object when dragged along a rough surface?
$(c)$ Relate power and work.
$(d)$ $A$ machine does $1960 \, J$ of work in $4 \, \text{minutes}$. What is the power?

$A$ porter moving vertically upstairs with a load on his head does work. Why?

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 $, m$)?