Question

The mass of the board 5 kg. What is the approximate mass of the boy?

Answer 1

$$ \begin{aligned} &m_{b} d_{b}+m_{\text {board }} d_{\text {board }}=m_{m} d_{m}\\ &m_{m}=80 \mathrm{~kg}\\ &m_{\text {board }}=5 \mathrm{~kg}\\ &d_{\text {board }}=1.5 \mathrm{~m} \text { (midpoint is at } 2.5 \mathrm{~m}, 1.5 \text { away from fulcrum) }\\ &d_{m}=1 m\\ &d_{b}=4 m\\ &80 \mathrm{~kg}^{*} 1 \mathrm{~m}=5 \mathrm{~kg}^{*} 1.5 \mathrm{~m}+4 \mathrm{~kg}^{*} \mathrm{~m}_{\mathrm{b}}\\ &m_{b}=18.1 \mathrm{~kg} \end{aligned} $$