Q: For a man who walks 720 m at a uniform speed of 2 m/s, then at a uniform speed of 4 m/s for 5 minute and then again in walks at a speed of 1 m/s for 3 minutes. His average speed is

Sol. Where s_{1} =720 m and t_{1} = s_{1}/v_{1} = 720/2 = 360 s

s_{2} = 4×( 5 × 60) = 1200 m , t_{2} = 300 s

s_{3} = 1× ( 3 × 60) = 180 m , t_{3} = 180 s

$\displaystyle v_{avg} = \frac{s_1 + s_2 + s_3}{t_1 + t_2 + t_3 } $

$\displaystyle v_{avg} = \frac{720 + 1200 + 180}{360 + 300 + 180 } $

= 2.5 m/s