Physics PDF - Travel Time Calculation

On a 120 km track, a train travels the first 30 km at a uniform speed of 30 km/h. How fast must the train travel the next 90 km so as to average 60 km/h for the entire trip ? In this numerical problem we have been given the total distance travelled by the train (which is 120 km), and the average spe...

On a 120 km track, a train travels the first 30 km at a uniform speed of 30 km/h. How fast must the train travel the next 90 km so as to average 60 km/h for the entire trip ? In this numerical problem we have been given the total distance travelled by the train (which is 120 km), and the average speed of the train for the whole journey (which is 60 km/h). From these two values we can calculate the total Gitil time taken by the train for the entire journey. This can be done as follows: We know that, Average speed = 60= So, Total time taken And, - Total distance travelled Total time taken t20 Total time taken l2o hours 60... (1) = 2 hours We will now calculate the time taken by the train for the first 30 km journey, and the next 90 km journey, separately (see Figure 17). (l) For the first part of the train journey, we have: = 30 krn/h Distance = 30 km Speed And, Time = Now, speed - 30= So, Timetaken= And, ? (To be calculated) Distance travelled Time taken 30 Time taken A hor., 30 = t hour gorm I n n 90 km x km/h -l 30 km/h T1 (2) Tt Tt n -l 'l n ft I n n L-l U L_t U L_t ltUUL.IUUU | || l I lt il || || || || 120 km 60 km/h tl J 1 Flgure t 7" Diagram for sample problem 4. (ll) For the second part of the train journey, let us suppose that the speed of the train is x km/h. So, for the second part of the train jonrner-, u-e har.e: Speed = And, 20 xkm/h Distance = 90 km Time = ? Motion (Su1.,posed) i lo be calcrilated)