$(a)$ State the law of conservation of energy.
$(b)$ Establish the relation between the $SI$ unit of energy and the commercial unit of energy.
$(c)$ $A$ toy car is displaced through $5 \, m$ on the application of a force of $7 \, N$. Assuming the force acts on the object throughout the displacement,what is the work done in this case?

(N/A) The law of conservation of energy states that energy can neither be created nor destroyed; it can only be transformed from one form to another. Thus,the total energy of an isolated system remains constant.
$(b)$ The $SI$ unit of energy is the joule $(J)$,and the commercial unit of energy is the kilowatt-hour $(kWh)$.
$1 \, kWh = 1 \, kW \times 1 \, h$
$1 \, kWh = 1000 \, W \times 3600 \, s$
$1 \, kWh = 3,600,000 \, J = 3.6 \times 10^6 \, J$.
$(c)$ Given: Force $(F)$ = $7 \, N$,Displacement $(S)$ = $5 \, m$.
Work done $(W)$ = $F \times S$
$W = 7 \, N \times 5 \, m = 35 \, J$.