Give reasons and mention the law related to these statements.

It is easier to push an empty box than to push the box full of books.

Give reasons and mention the law related to these statements.

It is difficult for a fireman to hold a hose which ejects large amount of water with high velocity.

Velocity of a moving body of mass $10 \mathrm{kg}$is changed from $3 \mathrm{m} {\mathrm{s}}^{-1}- 6 \mathrm{m} {\mathrm{s}}^{-1}$ in$3 \mathrm{s}$ by applying a force. Find the total work done.

Calculation of number of particles of$8 \mathrm{g}$of ${\mathrm{O}}_2$ molecules (number of molecules from mass) is given below:

i.e.,  $32 \mathrm{g} \mathrm{of} {\mathrm{O}}_{2 }= 1 \mathrm{mol} \mathrm{of} {\mathrm{O}}_2$

therefore,         $1 \mathrm{g} \mathrm{of} {\mathrm{O}}_2=\frac{1}{32}\mathrm{mol} \mathrm{of} {\mathrm{O}}_2$

                         $8 \mathrm{g} \mathrm{of} {\mathrm{O}}_2=\frac{8}{32} \mathrm{mol} \mathrm{of} {\mathrm{O}}_2$

Now, $1 \mathrm{mol} \mathrm{of} {\mathrm{O}}_2$$=6.022\times {10}^{23}$ particles of ${\mathrm{O}}_2$

therefore, $\frac{8}{32} \mathrm{mol} \mathrm{of} {\mathrm{O}}_2=\frac{8}{32}\times 6.022\times {10}^{23}$ particles of ${\mathrm{O}}_2$

                                           $=1.5055\times {10}^{23 }$particles of ${\mathrm{O}}_2$

Now, suggest an alternative solution to calculate the number of particles of $8 \mathrm{g}$ of ${\mathrm{O}}_2$ molecules.