$(a)$ 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?

$(D)$ The two conditions for work to be done are: $(i)$ $A$ force must act on the object, and $(ii)$ The object must undergo displacement in the direction of the force.
$(b)$ The work done by frictional force is negative because the force of friction acts in the direction opposite to the displacement of the object.
$(c)$ Power is defined as the rate of doing work. The relationship is given by $P = W / t$, where $P$ is power, $W$ is work done, and $t$ is time taken.
$(d)$ Given: Work $(W) = 1960 \, J$, Time $(t) = 4 \, \text{minutes} = 4 \times 60 = 240 \, s$.
Using the formula $P = W / t = 1960 / 240 = 8.166... \approx 8.17 \, W$.