91Ó°ÊÓ

The mass of a body is used to determine how much matter it contains. Kilogram is the SI unit of mass (kg). A person's mass does not alter over time.

The length of the see saw is $L=3.0\text{m}$.

The mass of the first child is $m_1=30.0\text{kg}$.

The distance of the first child from the pivot is $r_1=1.40\text{m}$.

The mass of the second child is $m_2=18.0\text{kg}$.

(a)

Let the distance of the second child from the pivot is${\text{r}}_{\text{2}}$. The equilibrium under torques gives,

$$\begin{gathered} m_1×g×r_1=m_2×g×r_2 \\ {r}_2=\frac{m_1×r_1}{m_2} \\ {r}_2=\frac{30.0×1.40}{18.0} \\ {r}_2=2.33\text{m} \end{gathered}$$

Hence, the distance is 2.33m.

(b)

The second child is more than the half of the total length of the see saw which is not possible. Hence, this is unreasonable.

(c)

If the mass of the second child were greater, the distance from the pivot would be less.

Hence, the reason behind the little fact is that the mass of the second child is small.