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 $, m$)?

$(a)$ Define one watt.
$(b)$ $A$ lamp consumes $1000 \ J$ of electrical energy in $10 \ s$. What is its power?

State the law of conservation of energy.

$A$ man drops a $10 \, kg$ rock from the top of a $20 \, m$ ladder. What will be its kinetic energy when it reaches the ground? What will be its speed just before it hits the ground? (Take $g = 10 \, m s^{-2}$)

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 $g = 10 \text{ m s}^{-2}$]

Four persons jointly lift a $250 \,kg$ box to a height of $1 \,m$ and hold it.
$(a)$ Calculate the work done by the persons in lifting the box.
$(b)$ How much work do they do just holding it?
$(c)$ Why do they get tired while holding it? $(g = 10 \,m/s^2)$