## College Physics (4th Edition)

(a) Superman's minimum constant acceleration should be $294~m/s^2$ (b) Superman's speed will be 266 m/s
(a) We can find the time it takes for the rock to fall 4.0 m: $\Delta y = \frac{1}{2}a_yt^2$ $t = \sqrt{\frac{2\Delta y}{a_y}}$ $t = \sqrt{\frac{(2)(4.0~m)}{9.80~m/s^2}}$ $t = 0.904~s$ We can find Superman's minimum constant acceleration: $\Delta x = \frac{1}{2}a_xt^2$ $a_x = \frac{2\Delta x}{t^2}$ $a_x = \frac{(2)(120~m)}{(0.904~s)^2}$ $a_x = 294~m/s^2$ Superman's minimum constant acceleration should be $294~m/s^2$ (b) We can find Superman's velocity when he reaches Lois Lane: $v_f^2 = v_0^2+2a_x\Delta x$ $v_f = \sqrt{v_0^2+2a_x\Delta x}$ $v_f = \sqrt{0+(2)(294~m/s^2)(120~m)}$ $v_f = 266~m/s$ Superman's speed will be 266 m/s