9-20 할리데이 11판 솔루션 일반물리학

$$\begin{cases} m_A&=2400\ut{kg}\\ m_B&=1600\ut{kg}\\ v_A&=80\ut{km/h}\\ v_B&=40\ut{km/h}\\ \end{cases}$$ $$r_\com=\frac{\Sigma rm}{M},$$ $$\begin{aligned} v_\com&=\dyt{r_\com}\\ &=\dt\(\frac{\Sigma rm}{M}\)\\ &=\(\frac{1}{M}\)\sum \(\dyt{r}m\)\\ &=\(\frac{1}{M}\)\sum \(vm\)\\ &=\frac{\Sigma mv}{M}\\ &=\frac{m_Av_A+m_Bv_B}{M}\\ &=64\ut{km/h} \end{aligned}$$