Answer to Question #128787 in Mechanics | Relativity for Shilpa S

Question

a train travels 50 km in half hour. it the stops at a station for 20 mins, before traveling for 2 hours at an average speed of 90 km/h what was the trains average speed over the whole journey

Answer 1

By definition, the average speed is:

$$v_{av} = \dfrac{l_{tot}}{T_{tot}}$$

where $l_{tot}$ is the total distance covered by the train in time $T_{tot}$.

The distance covered during the first part of the journey is 50 km, during the second part is 0 km and during the third part $90km/h\cdot 2h = 180$ km. Thus:

$$l_{tot} = 50 km + 0km + 180km = 230km$$

The time spent on the first part of the journey is 0.5h, on the second part $20/60 = 1/3 \space h$ and on the third part 2h, The total time is:

$$T_{tot} = 0.5h + 1/3\space h + 2h = 17/6 \space h$$

The average speed wil be:

$$v_{av} = \dfrac{l_{tot}}{T_{tot}} = \dfrac{230km}{17/6\space h} \approx 81.2 \space km/h$$

Answer. 81.2 km/h.