## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

(a) $t_1=\frac{d}{v} = \frac{50 ~mi}{40 ~mph}$ $t_1 = 1.25 ~hours$ $t_2=\frac{d}{v} = \frac{50 ~mi}{60 ~mph}$ $t_2 = 0.833 ~hours$ total time = $t_1 + t_2$ total time = 1.25 h + 0.833 h total time = 2.08 hours average speed = $\frac{100 ~mi}{2.08 ~hours}$ average speed = 48.1 mph (b) Let t be the time spent driving at each speed. total distance = 40t + 60t total time = 2t average speed = $\frac{40t+60t}{2t}$ average speed = 50 mph