7-42 할리데이 11판 솔루션 일반물리학

$$ \begin{cases} v_0&=0\\ m&=4.0\ut{kg}\\ t&=3.0\ut{s}\\ v&=12\ut{m/s}\\ \end{cases} $$ $$\ab{a}$$ $$K=\frac{1}{2}mv^2,$$ $$ \begin{aligned} W&=\Delta K=K~~(\because v_0=0)\\ &=\frac{1}{2}mv^2\\ &=288\ut{J}\\ &\approx 2.9\times10^2\ut{J} \end{aligned} $$ $$\ab{b}$$ $$v=v_0+at,$$ $$ \begin{aligned} a&=\frac{v-v_0}{t}\\ &=\frac{v}{t}\\ &=4\ut{m/s^2} \end{aligned} $$ $$ \begin{aligned} F&=ma\\ &=16\ut{N} \end{aligned} $$ $$P=\vec F \cdot \vec v,$$ $$ \begin{aligned} P_3&=16\cdot12\\ &=192\ut{W}\\ &\approx 1.9\times10^2\ut{W} \end{aligned} $$ $$\ab{c}$$ $$v=v_0+at,$$ $$ \begin{aligned} v(1.5\ut{s})&=(0)+4\cdot(1.5)\\ &=6\ut{m/s} \end{aligned} $$ $$P=\vec F \cdot \vec v,$$ $$ \begin{aligned} P_{1.5}&=16\cdot6\\ &=96\ut{W}\\ \end{aligned} $$