State the type of energy transformation in the following cases: $(a)$ in a loudspeaker $(b)$ solar battery.

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? $(g = 10 \,m/s^2)$

$A$ rocket is moving up with a velocity $v$. If the velocity of this rocket is suddenly tripled,what will be the ratio of the initial kinetic energy to the final kinetic energy?

$(a)$ What is meant by mechanical energy? State its two forms. State the law of conservation of energy. Give an example in which we observe a continuous change of one form of energy into another and vice-versa.
$(b)$ Calculate the amount of work required to stop a car of $1000 \, kg$ moving with a speed of $72 \, km \, h^{-1}$.

$A$ light and a heavy object have the same momentum. Find the ratio of their kinetic energies. Which one has a larger kinetic energy?

Can any object have momentum even if its mechanical energy is zero? Explain.