Define work. Write an expression for work in terms of force and displacement. State the $SI$ unit of work done.

$A$ weightlifter lifts $240 \,kg$ from the ground to a height of $2.5 \,m$ in $3 \,s$. The average power is:

$(a)$ Define the term 'average power'.
$(b)$ When a player hits a football,it moves along a curved path and then falls to the ground. Calculate the work done by the force of gravity on the football.
$(c)$ Two friends,Ram and Shyam,each having a mass of $40 \, kg$,go for rock climbing. Ram climbs to a height of $3 \, m$ in $10 \, s$ and Shyam covers the same height in $12 \, s$. Is the work done by Ram and Shyam equal or not? Compare the power of Ram and Shyam.

$(a)$ 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?

$(a)$ Is it possible that a body is in accelerated motion under the action of a force,yet no work is being done by the force? Explain with an example.
$(b)$ Two bodies of masses $m_{1}$ and $m_{2}$ have equal kinetic energies. What is the ratio of their linear momenta?

We wind our watch once a day,what happens to the energy?