Name the practical unit of power in engineering.

Justify,giving proper reasoning,whether the work done in the following case is positive or negative: Work done by an applied force on a body moving on a rough horizontal plane with uniform velocity.

$(a)$ 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?

Calculate the power of an engine required to lift $10^{5} \text{ kg}$ of coal per hour from a mine $360 \text{ m}$ deep. [Take $g = 10 \text{ m s}^{-2}$]

One joule of work is said to be done when:

An iron sphere of mass $30 \, kg$ has the same diameter as an aluminium sphere whose mass is $10.5 \, kg$. The spheres are dropped simultaneously from a cliff. When they are $10 \, m$ from the ground,they have the same: